Large parts of the rewrite came from contributors who had never worked on fish before.

That's pretty useful alone.

And there's this:

Thread Safety

Allowing background functions and concurrent functions has been a goal for many years. I have been nursing a long-lived branch which allows full threaded execution. But though the changes are small, I have been reluctant to propose them, because they will make reasoning about the shell internals too complex: it is difficult in C++ to check and enforce what crosses thread boundaries.

This is Rust's bread and butter: we will encode thread requirements into our types, making it explicit and compiler-checked, via Send and Sync. Rust will allow turning on concurrent mode in a safe way, with a manageable increase in complexity, finally enabling this feature.

They did it "for the vibes"

Vibes are just as important to free/open source software as proprietary software and although there were solid technical reasons for the port, the PR outcomes are added benefits.

It was needed to safely further support for concurrent features? If they follow through on adding that support, there will likely be adoption.

The problem is in most cases the implementers stop at "same thing but in rust" without taking advantage of that.

I can't fully blame them since just duplicating an existing thing is a huge undertaking.