I wonder if a better use of time would be to just focus on making mypy faster. We already have too many tools, and it's often better to just focus on making the existing stuff better rather than developing a new tool (relevant XKCD [1]). There's gotta be ways to make mypy faster, even if it takes some breaking changes.
One consideration there would probably be how many committal decisions mypy has made. One of the main reasons a new tool might be better than an old one is that it can be built fresh, with all the lessons learned but (essentially) none of the baggage of bad design decisions.
Another thing I've also weighed is that developing any kind of static analysis tool whose source code is not in the language it analyzes (such as Ruff) immediately blocks you off from a large portion of maintainers. Your maintainers cannot be your end users unless they work with both Python and Rust. While Python is definitely one or more magnitudes slower than Rust, I can't imagine that mypy is at the point where the language choice itself is the only bottleneck.