I think you vastly overestimate the separability of these systems.
Picture 10,000 lines of code in one method, with a history of multiple decades.
Now picture that that method has buried in it, complex interactions with another method of similar size, which is triggered via an obscure side-effect.
Picture whole teams of developers adding to this on a daily basis in realtime.
There is no "meaningful progress" to be made here. It may offend your aesthetic sense, but it's just the reality of doing business.
The alternative is to continue with a process that's been demonstrably successful, despite it offending your sensibilities.
Banks are prepared to pay for it. People are prepared to do it. It meets the business needs. Change is massively high-risk in a hugely conservative industry.