Firstly yes, not all Linux targets are supported Rust targets, as a result if you were to rewrite Linux memory management code in Rust you can't ship that without breaking some Linux platforms. Over time it is expected that: Linux will stop caring about some very, very old targets; Rust will gain support for some more targets (especially those that aren't very, very old) and so this issue goes away. Meanwhile drivers cannot run on all targets so them being in Rust doesn't change whether your target can run Linux.
Rust for Linux was driven by existing Linux developers. So they obviously think that's great news for Linux not just for Rust. One reason is: Nobody else was delivering a viable way forward. If there were ten of these safe low-level languages and by doing nothing Linux could be sure one would pick "Make ourselves suitable for Linux" as its goal then they could just sit back. Instead Rust is the only game in town, so it's either adjust Rust [ e.g. Rust for Linux alloc crate doesn't have the concatenating + operator on String like your userspace Rust does because that's the sort of thing which makes Linus angry ] or risk not having any safe path forward for the foreseeable future.
And yes, ultimately whether Rust for Linux goes into the Linus tree at some point is always ultimately a decision for Linus Torvalds. It's just that he made friendly noises about it, and there are people putting the work in, so it's like for dozens of other prospective Linux features, we can assume it will land but we shouldn't assume when.
