But to get the most value out of Rust, one needs to use the Cargo ecosystem too (even for kernel code!) and that will be a much bigger political quagmire.
Probably one good end state is if the core kernel exposes appropriate safe abstractions, and vendors with out-of-tree modules (which tend to be worse at security than the main kernel) could use those APIs + things from crates.io to build their modules.
But yes, figuring out how to use useful things that aren't in libcore is one of the open questions.
But to get the most value out of Rust, one needs to use the Cargo ecosystem too (even for kernel code!) and that will be a much bigger political quagmire.