the port does not have any immediately benefits to end-users or gives us any new special features. However, it improves the future maintainability and bug-fixing capability of the dev team compared to the C++ base.

I remember them also specifically talking about an issue regarding asynchrony. If I remember correctly, it was that their current job system doesn't actually execute background jobs in parallel. They even had a massive pull request ready to fix this in C++, but did not feel confident in it not introducing a ton of bugs.

But the most important reason is fun. Fish is a hobby project. The core team wanted to switch to Rust, because it would be more fun. This also resulted in tons of new contributors suddenly offering help. So many people trying to find "rational" explanations like you'd expect them from a company, when completely different dynamics apply in a volunteer project.