Fall back or Fail
There are worse things to fall back on than a system that's working, even if it's a 30 year old mainframe running COBOL.
Mainframes can be upgraded. COBOL can be migrated almost anywhere (there's even a FOSS version). And COBOL is easy to learn, albeit tedious to code.
My advice would be to first define what your legacy system doesn't do and what it would cost to update it so it could do that.
Biting the hand that feeds IT
