Anyone who uses intmax_t in a durable API (public function argument, member of public structure) is just a goof.
How would you compile that to machine code which would, when run, still give you the maximum integer on next year’s CPU with all-new super-duper wide integers?
It is more reasonable to think of a binary to be compiled to a specific architecture triplet, and if the CPU changes, the architecture must change, and therefore the ABI, and if the OS wants to run a binary from an older architecture, some emulation layer is needed.
Of course, this would be a lot of work for the operating system people if they want to run binaries from lots of what are now different architectures, and it was apparently easier to just force the C standard to abandon the entire concept of intmax_t being the largest.
intmax_t by intent is the maximum integer available on a given platform. Abandoning that idea is a disservice to compiler writers, programmers -- especially scientific programmers -- everywhere.
That would add a lot of friction to changing CPUs, and making distributing software more complicated.