About a decade ago I looked into moving some COBOL components off-mainframe (either as COBOL-on-Linux or a rewrite into Java, which itself is really COBOL Mk II at this point), and your point about the operating environment is one of the key elements, but not all of it; there's also the fact that the first big shift to automation, via mainframe assembler and COBOL, is when companies sacked a lot of the folks who knew how and why the pre-automation processes worked - that knowledge exists in the mainframe code and the heads of the people who work(ed) on it, and nowhere else. A rewrite or a replatform is very, very hard and risky as a result; the system is now defined by how the mainframe runs the processes, to a very large degree.

The third is that COBOL is only the tip of the iceberg. As soon as I spent time learning about the code I was being asked to look at, you get into decades of evolving programming practises. Modern COBOL is multithreaded, probably uses DB2 and relational datamodels. COBOL from thirty years ago is probably single-threaded, only runs right on high-clocked single-execution models, cuts down to hand-written s390 assembler regularly, and uses VSAM files with non-relational data. Older code still will be sharing data simply by banging it into memory regions for other code to read out of, because that's how you got performance back in the day.

Trying to identify how you'd pull a function out of that and move it off is somewhere between extremely difficult and impossible. It's usually so complicated and expensive it's easier to try and hire people who want to apprentice as mainframe programmers and keep the current codebase running.

> A rewrite or a replatform is very, very hard and risky as a result; the system is now defined by how the mainframe runs the processes, to a very large degree.

And that's why so many neo-banks/fintechs are eating the lunch of the established banks left and right, same for insurance. The "old guard" is unwilling to pay the costs of not just upgrading off of mainframes (aka the rewrite work itself)... but of changing their processes. That is where the real cost is at:

When you have 213.000 employees like BoA has and everyone needs to have at least 10 hours of training and 2 weeks until they're familiar with the new system enough to be fully productive, that's like 2 million man-hours just for training and 16 million hours in lost productivity, so assuming $50/h average salary it's around 900 million dollars in cost. Unfortunately for the dinosaurs, the demands of both the customers and (at least in Europe) regulatory agencies especially for real-time financial transfers just push the old mainframe stuff to limits, while at the same time banks don't want to cede more and more of that cake to Paypal and friends that charge quite the sum for (effectively) lending money to banks.

In contrast, all the newcomers start with greenfield IT, most likely some sort of more-or-less standard SAP. That one actually supports running unit and integration tests automatically, drastically reducing the chance of fuck-ups that might draw in unwanted regulatory attention.

BOA doesn't train the vast, vast majority of its workforce on mainframe systems these days. No one working in a branch or call center is looking at green screens anymore. The mainframe systems are simply used as back-ends connected through web services (yes, even in CICS!) or MQ Series and the like to web GUIs.

Source: worked there for many years, and built some of those integration systems.

Eh, I think the tech stack is less important than the legal and regulatory structure.

Most fintechs aren't banks and partner with a Real Bank™ to provide the actual bank accounts. Fintechs are under much less regulatory scrutiny (for now—that may be changing with recent, high-profile screwups) and can move with much more freedom regardless of the tech stack they've chosen.

Migrations are still a thing, with various approaches and success rates.