I think this is a key point. Old tech running perfectly fine; if it ain't broken don't fix it.
There's no way to be fully confident that what appears to be a simple update might not break the whole system. And I'd imagine this is likely what the bank is wanting to avoid rather than having to pay programmers to implement the changes.
You only have to look as far as TSB for a cautionary tale.
But it is broken, as it is unable to support frequent European names.
There's also the case to be made that if the sources are lost, you need to rewrite from scratch, as you never know when you need to implement new functionality that business deems critical. If you only start the new implementation after business is breathing down your neck, you are in a world of hurt.
There's no way to be fully confident that what appears to be a simple update might not break the whole system. And I'd imagine this is likely what the bank is wanting to avoid rather than having to pay programmers to implement the changes.
You only have to look as far as TSB for a cautionary tale.