If nobody can understand the application, it cannot be modified.
If an application cannot be modified, its value will continuously decrease with time.
That decrease in value, as well as the projected cost of building a replacement, is tantamount to accruing interest on a debt.
They could have reduced that debt by moving to functionally equivalent but more commonly taught languages back when their team could still understand the system.
It wasn’t necessarily wrong for them to accrue this debt - after all, banks and these monolithic institutions are masters at effectively utilizing debt. However, the practice may have become habitual and it looks like they might be in pretty deep at this point.
What you’re describing is a management and leadership failure.
In US tech we have an overabundance of Type A people in it for the money, but there are plenty of smart Type Bs who just want stability and a life, for a fraction of the going SV engineering pay.
You have to wonder why the latter is impossible for American companies to achieve.