From their blog post:

Finally, subjectively, C++ isn’t drawing in the crowds. We have never had a lot of C++ contributors. Over the 11 years fish used C++, only 17 people have at least 10 commits to the C++ code. We also don’t know a lot of people who would love to work on a C++ codebase in their free time.

Hard to tank when you don’t have many to begin with. Rust is far nicer to new users to contribute to then old C++ code. Which can be seen in their github - in the last 24 months 16 people have contributed more then 10 commits. Which is during the conversion period - I dont expect that many of those to be C++ contributions. So rust does not seem to have hurt their contributions at all and in fact looks to have helped.