What you're missing is, mainly, the so-called "Trusting Trust" attack, where a compiler is backdoored such that whenever it's used to compile another compiler,

What you're missing is, mainly, the so-called "Trusting Trust" attack, where a compiler is backdoored such that whenever it's used to compile another compiler, the same backdoor is inserted into that compiler. Thus even assuming good intentions on the part of the current Rust maintainers, a backdoor could hypothetically have been inserted into version X's official compiler binaries at some point, which then infected version X+1, which infected version X+2, and so on. I say hypothetically, because IIRC Rust's bootstrap chain has been replicated, including with mrustc, resulting in identical binaries. There is absolutely no need to re-replicate it on every random architecture. (And even if you did want to do so, qemu already exists and can emulate x86, though wasm might be slightly faster.) But some people have strange ideas about how bootstrapping ought to work...