You're absolutely right that switching languages alone doesn't solve the problem. The real issue isn't COBOL itself but the deep entanglement of business logic with the mainframe ecosystem, things like CICS, IMS, and even the way data is stored and processed. But I still think there's a path forward, and I’ll share a thought experiment based on my experience working alongside colleagues who’ve spent years maintaining these systems.

I’ve seen firsthand how much frustration COBOL can cause. Many of my colleagues didn’t enjoy writing it, they stuck with it because it paid well, not because they loved the work. The language itself isn’t the hard part; it’s the decades of accumulated technical debt and the sheer complexity of the environment. Over time, these systems become so intertwined with business processes that untangling them feels impossible. But what if we approached it incrementally?

Imagine taking an existing COBOL codebase, say, for a large insurance system and identifying the core business logic buried within it. These are the rules and conditions that power critical operations, like calculating premiums or processing claims. Now, instead of trying to rewrite everything at once, you build a parallel backend in a modern language like Java or Go. You don’t aim for a literal translation of the COBOL code, you focus on replicating the functionality in a way that makes sense in a modern context. For example, replace hardcoded file operations with database calls, or screen based interactions with REST APIs.

Most mainframe customers already use middleware like MuleSoft or IBM Z/OS Connect to route requests to both systems simultaneously. For every write operation, you update both the mainframe’s DB2 database and a modern relational database like Postgres. For every read operation, you compare the results from both systems. If there’s a discrepancy, you flag it for investigation. Over time, as you handle more and more business scenarios, you’d start covering all the edge cases. This dual system approach lets you validate the new backend without risking critical operations.

Of course, this process isn’t without its struggles. Testing is a huge challenge because mainframe systems often rely on implicit behaviors that aren’t documented anywhere. My colleagues used to joke that the only way to understand some parts of the system was to run it and see what happened. That’s why rigorous testing and monitoring are essential you need to catch discrepancies early before they cause problems. There’s also the cultural side of things. People get attached to their mainframes, especially when they’ve been running reliably for decades. Convincing stakeholders to invest in a multi year migration effort requires strong leadership and a clear case for ROI.

But I think the effort is worth it. Moving off the mainframe isn’t just about saving money though that’s a big part of it. It’s about future proofing your organization. Mainframes are great at what they do, but they’re also a bottleneck when it comes to innovation. Want to integrate with a third party service? Good luck. Need to hire new developers? Most of them have never touched COBOL. By transitioning to a modern platform, you open up opportunities to innovate faster, integrate with other systems more easily, and attract talent who can actually work on your codebase.

In the end, this isn’t a quick fix it’s a long term strategy. But I believe it’s achievable if you take it step by step. Start small, validate constantly, and gradually build up to a full replacement. What do others think? Are there better ways to tackle this problem, or am I missing something obvious?

I don't think you're missing anything fundamental, but I've worked with systems written in Fortran, C, C++ and Python that have the same problems. I suspect the systems I'm working on in Python & Rust will have the same issues if they last 10+ years.