But requirements change, so upgrades are a normal part of business, due to at the bare minimum legal reporting requirements changing, even if there weren't all

But requirements change, so upgrades are a normal part of business, due to at the bare minimum legal reporting requirements changing, even if there weren't all sorts of new things like internet banking making their old APIs inadequate. If the software isn't extendable or changeable then it isn't 'working fine', because if you can't offer internet banking or do FATCA reporting you go out of business or get shut down.