“Over the 11 years fish used C++, only 17 people have at least 10 commits to the C++ code.”
Fish went from 57K lines of C++ to 75K lines of Rust in 2,604 commits over almost two years. Seven people did most of the work. The price is Cygwin support, gone because Rust has no Cygwin target. The real reason for the rewrite sits in that quote. Nobody wants to touch C++ in their free time.