COBOL is not about the language, it's about the whole environment in the mainframe. LLM fanboys won't understand this. You need something like a mainframe with a 99.99% uptime no matter what happens in hardware, with live CPU swapping and such.
Erlang/Elixir/OTP has a resilient environment with hot swapping. Is that truly problematic?
Yes and that mainframe runs several other technologies besides COBOL, and more importantly, that don't depend on it (COBOL fanboys won't understand this ;) )
Given the option, I would go with DB 2 on z/OS instead of COBOL, for example.
Can you explain why you need one machine never falling over ever instead of a cluster of machines never falling over at once collectively?
You don't. Either would work. Getting rid of a system running cobol on a mainframe is perhaps the hardest work possible in software engineering.