> I guess young professionals do not want to write software that is very simple and works properly (/s, but only partially)

Maybe it makes sense to look at this through the developers' career-management lense.

Specializing in a potentially niche technology such as mainframe COBOL means limited job options as time goes on.

With few employers and few employees in that market, small changes could drive salaries much higher or much lower. (I'm speculating.)

It would make sense for developers to demand higher salaries to justify that risk, but AFAIK banks have a reputation for low developer pay.

Limited is understatement. Extremely limited, singli digit options.