> Witness Windows, which has little to no problem with multiple libcs in a process

Only so long as you don't pass data structures from one to the other. The same caveats wrt malloc/free or fopen/fclose across libc boundaries still applies.

Well, not anymore, but only because libc is a system DLL on Windows now with a stable ABI, so for new apps they all share the same copy.

Yes, but in a culture where this kind of thing is normal (and statically linking the libc was popular for a while), that is in mostly understood, CPython’s particular brand of awfulness notwithstanding. It is in any case a much milder problem than two libcs fighting over who should set the thread pointer (the FS segment base), allocate TLS, etc., which is what you get in a standard Linux userspace.