> Heck, GTK+, which is used by millions, had just one person working on it full time a few years ago (not sure if changed since), and lamenting how there's no community help in the project's blog. And that's for a hugely used and know project. I doubt a Rust UI lib would get more than that.

This is not true for GTK+ 3. GTK+ 2, however, has been in maintenance mode since the introduction of GTK+ 3 and has been in that state since.