Fuchsia is using Rust for some pieces. That's what I assume MS would do. MS is a company with tons of products, not just an OS, and certainly not a single monolith. You wouldn't rewrite everything in a day, you would slowly move things over time.

Linus is never going to switch to anything else other than C, UNIX is married to C.

So it is always going to be something else, not UNIX related.

Fuchsia uses Rust and Go in several areas.

Microsoft is using Rust on VSCode, Azure and IoT Core.

Fuchsia implements TCP/IP stack in Go. It is an improvement over others.