Are you operating under the assumption that the largely implicit support tiers in other compilers are better? In other words: do you think GCC’s m68k backend (to pick an arbitrary one) has been as battle-tested as their AArch64 one?
(I think the comment about evangelists is a red herring here: what Rust does is offer precison in what it guarantees, while C as an ecosystem has historically been permissive of mystery meat compilers. This IMO doesn’t scale well in a world where project maintainers are trivially accessible, since they have to now field bug reports on platforms they can’t reproduce for and never intended to support to begin with.)