> It seems unlikely [Apple] would throw away that investment and move to Rust.

Apple has invested in Swift, another high level language with safety guarantees, which happens to have been created under Chris Lattner, otherwise known for creating LLVM. Swift's huge advantage over Rust, for application and system programming is that it supports an ABI [1] which Rust, famously, does not (other than falling back to a C ABI, which degrades its promises).

[1] for more on that topic, I recommend this excellent article: https://faultlore.com/blah/swift-abi/ Side note, the author of that article wrote Rust's std::collections API.