Well. If you put a large glass window on the reinforced steel safe to make sure you can observe inside the safe. You can’t exactly expect criminals to not just smash window instantly to take everything instead of struggling to open the safe harder way.
Making master key is also not the approach that works because unlike physical keys, digital keys can be copied millions of times exactly without any flaw over miliseconds without requiring any specialized tool on site.
COBOL case is bit different. You can’t just modernize millions of lines of code that is functionally unique without service disruption - and services that uses COBOL that large often tends to be very sensitive.
The fact that COBOL as a language is both atrocity to either use or read didn’t help that either.