Well, that's supposedly the advantage of a standardized language. If the Poly/ML maintainer gets hit by a bus, code that only uses the core language and basis (the latter is quite extensive) should compile under a different compiler like SML/NJ or mlton. In practice of course it wouldn't be that easy, compiler-specific code would need to be ported, but it's good practice to isolate this in specific modules anyway.
FYI, I picked Poly/ML because of two things: native-code compiler that can target Apple M1 CPU; and thread support.
The other possibility is to take over maintenance. I took a quick look, and the code is readable enough that submitting patches for point bugfixes or portability problems looks feasible. Working on the core compiler would require a lot of background reading though, may or may not make sense from a cost/benefit analysis.