Decades old might definitely mean duct taped and unstable. In about 1995 I witnessed the beginning of the end of at the time very successful company that just had tripled their whole organisation. But the code base was 5 million lines of a lot of copy pasted literal spaghetti code. And only a handful of ageing coders that had been with the company almost from the start could get anything done. And then, one day, those damn customers demanded a Windows application, instead of the classy DOS application.