Rust compiler support works differently

I think the more plausible argument is that having a newer Rust compiler available but using the older one instead doesn't introduce any new bugs (or any new code that you're running), and building a new kernel with an old Rust compiler isn't any more stable than building a new kernel with a new Rust compiler. In the C userspace world, it's potentially a problem to build some things with a new compiler and other things with an old compiler, so you don't necessarily want to have a bunch of different versions available, but Rust likes to build all of the source that goes into an executable (including the standard library) when you build it, and the kernel does this too, even for C.

It's probably best not to rebuild python-cryptography and curl with a new Rust compiler release just because it exists, but that doesn't mean you can't build your kernel with a newer Rust compiler when that's what people are doing upstream.