> business process, ... that might date back decades
The thing about business processes is that they change frequently. Businesses are about competing with others and trying to increase their revenue and profit. They will change the way they set prices, discounts, they will change the product they sell, the way that they sell them. If they don't do that then their competitors will and will take business from them. The computer systems are required to be amended to implement these changes.
> rewriting the code in a modern language
Back in the 80s there was a Datamation survey that reported that 80% 9or so) of COBOL programming was reported as being 'maintenance' whereas 90% of C programming was new applications. Some seemed to conclude that if COBOL programs were converted to C then the 'maintenance overhead' would disappear and the programmers would create new applications.
The whole survey was seriously flawed. The COBOL and C sites were doing completely different types of applications. The COBOL 'maintenance' was, in fact, updating the business applications to handle changes in the business processes as the businesses re-invented themselves to compete better.