Fish was rewritten for a different reason. Essentially, fish wanted to do more with threads, and making something that worked well was going to involve a large amount of work to ensure it was safe and correct (you don't want data races in your shell). The main dev and a lot of the contributors were already familiar with Rust, so it made a lot of sense.

And so far, Linux seems to be taking it really slowly. There are drivers in Rust, as well as an optional scheduler. The main language for kernel development is still C.

Rust may not be the best option for all projects, but it can be integrated in with minimal disruption if projects do find value with it.