Yeah, I enjoyed when fish shell did their rewrite and people were arguing whether it makes sense technologically, like it’s a mature codebase, so it’s not likely to reduce the number of bugs and yadda yadda yadda.

None of this mattered, because fish is developed by a group of hobbyists. They didn’t want to continue developing in C++, because it was a pain in the ass. And for sure, one of the reasons they wanted to switch to Rust is because it’s new and exciting.
These reasons are perfectly valid, because they’re volunteers. If they’re not having fun, then they will just developing.