It might be helpful to consider the Firefox situation, too. That is also a relatively large, established, and actively-developed software system that began to include some Rust code later on in its existence.

If I'm not mistaken, the first stable Firefox releases including notable usage of Rust code came out in late 2017. So that's about 6 years of Rust in Firefox.

From a Firefox user's perspective, I can't say that I'm impressed by it.

Building Firefox from source is more involved than it was before. As you mentioned, additional tooling for Rust is required in such a situation.

I haven't really noticed any real Firefox feature, functionality, or performance improvements that I could directly attribute to the use of Rust.

It also doesn't seem like using Rust has allowed the Firefox developers to be significantly more efficient or productive than they were before.

Maybe an argument could be made that some security issues, for example, have been avoided thanks to Rust, but that's not really provable in any meaningful way.

After seeing the Firefox situation, my expectations for the use of Rust in the Linux kernel are pretty low.