If you read my post, one needs to use them to contribute to them. Past a certain point that matters less; early on, it matters a lot.

This is true, but for a compiler project, I think compiler expertise is a much bigger gating factor than language expertise. If you want to maximize effective contribution, you're probably better off picking a language that lots of compiler hackers know.

There's more to this project than compilers.

As an example, I've written custom TSLint plugins before. I'm not writing those in Rust.

There's a lot of wisdom in sticking to your lingua franca until you prove you can't.