I agree with your main point, although I think your example of COBOL being used to this day in financial institutions is actually the opposite problem. The guys that originally developed that shit were damn good programmers, but they were severely constrained by the available hardware, limitations of the language, etc. So they had to get really clever in order to make these massive, complicated systems work. In my experience, those really old legacy systems tend to be rock solid with near 100% uptime and almost no errors. They've never been rewritten because doing so would be a multi-year effort costing millions of dollars, and the end result would be a system that is most likely slower, buggier, and has less functionality.

TLDR: The old COBOL systems are unmaintainable messes not because of incompetent developers, but because the limitations of the available technology when they were originally developed forced a bunch of really good devs to have to get extremely creative and hacky with their solutions.