The Full Rewrite Myth: Why Greenfield Projects Turn Into Legacy Nightmares by Month 18

Every enterprise architect has heard the siren song of the "fresh start." You look at a 15-year-old COBOL-backed monolithic web app, see the layers of spaghetti code and undocumented "business logic," and decide the only path forward is to burn it down and start over. This is the full rewrite myth greenfield trap—the belief that starting from scratch will somehow bypass the complexities that made the original system messy in the first place.

According to Replay’s analysis, 70% of legacy rewrites fail or significantly exceed their original timelines. By the time you reach Month 18 of a greenfield project, the team is usually exhausted, the budget is depleted, and you’ve managed to recreate only 60% of the original functionality—while introducing a new set of bugs that didn't exist in the legacy system.

TL;DR: Greenfield rewrites are often a $3.6 trillion technical debt trap. Most fail because they underestimate the "hidden" logic in undocumented legacy systems. Replay offers a "Third Way": Visual Reverse Engineering. By converting video recordings of legacy workflows into documented React code, enterprises save 70% of modernization time, moving from an 18-month roadmap to a matter of weeks.

The Economic Reality of the Full Rewrite Myth Greenfield Approach#

The allure of greenfield development is psychological, not technical. Developers want to use the latest stack (Next.js, Tailwind, GraphQL), and stakeholders want a modern UX. However, the full rewrite myth greenfield strategy ignores the fact that legacy systems are not just "old code"—they are the physical manifestation of a decade of edge cases, regulatory compliance, and bug fixes.

Industry experts recommend looking at the "Replacement Cost" versus the "Modernization Cost." When you choose a full rewrite, you are paying to rediscover business rules that your company has already paid for once.

The $3.6 Trillion Problem#

Global technical debt has ballooned to an estimated $3.6 trillion. For a typical Financial Services or Healthcare firm, maintaining these systems consumes 80% of the IT budget. When leadership tries to break this cycle with a greenfield project, they often hit the "Month 18 Wall."

The Month 18 Wall is the point where:

1. •The "new" system is now 18 months behind on feature parity.
2. •The "old" system has had 18 months of critical security patches and regulatory updates that weren't captured in the new requirements.
3. •The original architects of the greenfield project have started to leave, taking the "new" tribal knowledge with them.

Modernizing Legacy Systems requires a shift from "Replacement" to "Visual Reverse Engineering."

Why Documentation is the Silent Killer#

Industry data shows that 67% of legacy systems lack any form of usable documentation. When you embark on a full rewrite myth greenfield journey, your developers are essentially acting as archaeologists. They spend 40% of their time reading old code (or worse, guessing) and only 20% writing new code.

Video-to-code is the process of recording a user performing a specific workflow in a legacy application and using AI-driven automation to transform those visual interactions into clean, documented React components and state logic.

By using Replay, teams bypass the "archaeology" phase. Instead of manual discovery, Replay’s Visual Reverse Engineering platform captures the "truth" of the UI and the underlying flows, generating a Design System and Component Library directly from the source of truth: the running application.

Comparison: Manual Rewrite vs. Replay Automation#

Breaking the Cycle with Visual Reverse Engineering#

To avoid the full rewrite myth greenfield trap, architects must stop treating the UI as a separate entity from the business logic. In legacy systems, the UI is the documentation of the business process.

Visual Reverse Engineering (VRE) is a methodology where the existing user interface serves as the blueprint for the new system. Instead of writing requirements in Jira and hoping the developer interprets the legacy behavior correctly, Replay records the actual "Flows."

How Replay Transforms the Workflow:#

1. •Record: A subject matter expert (SME) records a standard workflow (e.g., "Onboarding a new insurance claimant").
2. •Analyze: Replay’s AI Automation Suite identifies UI patterns, form logic, and navigational structures.
3. •Generate: Replay produces a documented React library and a "Blueprint" of the application flow.
4. •Export: Developers get clean, SOC2-compliant TypeScript code that mirrors the legacy behavior but uses modern best practices.

Implementation: From Legacy Spaghetti to Clean React#

Let’s look at what a typical "Month 18" nightmare looks like in a manual rewrite versus the clean output generated by Replay.

The Legacy Mess (Conceptual)#

In the old system (perhaps a JSP or WinForms app), logic is tightly coupled with the DOM, making it nearly impossible to extract without breaking things.

typescript
// The "Archaeology" phase: Trying to guess what this does
function process_data_v2(e) {
  // 150 lines of undocumented conditional logic
  if (e.target.value === 'A1' && global_user_role === 4) {
    document.getElementById('submit-btn').style.display = 'block';
    // Why is this here? Nobody knows.
    doLegacyValidation(); 
  }
}

The Replay Output (Modern React)#

Replay extracts the intent of the interaction and generates a clean, modular component. This allows you to maintain the full rewrite myth greenfield dream of modern code without the 18-month discovery period.

tsx
import React from 'react';
import { useClaimForm } from './hooks/useClaimForm';
import { Button, Input, Card } from '@your-org/design-system';

/**
 * Extracted from Legacy "Claimant Information" Screen
 * Recording ID: rec_987654321
 * Replay Blueprint: Insurance-Flow-v1
 */
export const ClaimantForm: React.FC = () => {
  const { data, updateField, isValid, submit } = useClaimForm();

  return (
    <Card title="Claimant Information">
      <div className="grid grid-cols-2 gap-4">
        <Input 
          label="Claimant Name"
          value={data.name}
          onChange={(val) => updateField('name', val)}
          required
        />
        <Input 
          label="Policy Number"
          placeholder="XX-000000"
          value={data.policyNumber}
          onChange={(val) => updateField('policyNumber', val)}
        />
      </div>
      <Button 
        variant="primary" 
        disabled={!isValid} 
        onClick={submit}
      >
        Continue to Documentation
      </Button>
    </Card>
  );
};

By generating the Design System and Component Library directly from recorded sessions, Replay ensures that the "new" code is actually consistent with the "old" business rules.

The "Flows" Feature: Mapping the Enterprise Architecture#

One of the biggest reasons the full rewrite myth greenfield fails is the loss of complex navigation logic. Large-scale systems in Government or Manufacturing have thousands of "if-then-else" paths.

According to Replay's analysis, manual mapping of these flows takes months. Replay’s "Flows" feature automates this by visualizing the entire application architecture based on user recordings. It creates a living map of how data moves from Screen A to Screen B.

Visualizing Application Flows:

• •Blueprints: The architectural map of the legacy app.
• •Library: The atomic components extracted from the UI.
• •AI Automation Suite: The engine that converts visual elements into functional code.

For organizations in regulated industries, Replay is built for high-security environments. It is SOC2 and HIPAA-ready, and for those in Government or Defense, it can be deployed On-Premise to ensure that sensitive data never leaves the internal network.

Scaling Beyond the Pilot#

Once you realize the full rewrite myth greenfield is a path to failure, you can adopt a "Strangler Pattern" powered by Replay. Instead of waiting 18 months for a big-bang release, you can modernize one "Flow" at a time.

1. •Month 1: Record the top 10 most critical user flows.
2. •Month 2: Use Replay to generate the React components and Design System.
3. •Month 3: Deploy the first modernized module into production, running alongside the legacy system.

This incremental approach reduces risk and provides immediate ROI. Instead of the 40 hours per screen required for manual reconstruction, Replay reduces the effort to 4 hours. In a 500-screen enterprise application, that is the difference between a 20,000-hour project and a 2,000-hour project—a 90% reduction in labor costs.

Building an Enterprise Component Library is no longer a multi-year manual effort. With Replay, it becomes an automated byproduct of your discovery process.

Frequently Asked Questions#

Does Replay work with legacy technologies like Silverlight, Flash, or Mainframe screens?#

Yes. Because Replay uses Visual Reverse Engineering, it doesn't matter what the underlying "black box" technology is. If a user can see it on a screen and interact with it, Replay can record it, analyze the UI patterns, and convert those patterns into modern React code.

How does Replay handle complex business logic that isn't visible on the screen?#

Replay excels at capturing the "UI State" and "Interaction Logic." For deep backend business logic (like complex database calculations), Replay provides the "hooks" and "Blueprints" that allow developers to easily connect the modern UI to existing APIs or microservices. It bridges the gap between the user experience and the data layer.

Is the code generated by Replay maintainable or is it "AI spaghetti"?#

Replay is built for Senior Enterprise Architects. The output is clean, modular TypeScript that follows modern best practices. It generates a structured Design System and uses standard React patterns (like Hooks and Functional Components). Unlike generic AI code generators, Replay's AI Automation Suite is specifically tuned for enterprise UI patterns.

Can we use Replay for on-premise legacy systems that cannot access the public cloud?#

Absolutely. Replay offers an On-Premise deployment model specifically for Government, Financial Services, and Healthcare sectors that require strict data residency and security compliance. We are SOC2 and HIPAA-ready.

How does Replay compare to traditional "Low-Code" platforms?#

Low-code platforms often lock you into a proprietary ecosystem. Replay is the opposite; it generates standard, open-source React code that you own entirely. You aren't building "on" Replay; you are using Replay to accelerate your move to a standard modern stack.

Ready to modernize without rewriting? Book a pilot with Replay