I don't think that Tor in javascript would improve anything. Tor in golang on the other hand would gain multithreading and ease of integration. It would drop in nicely with the Golang standard library.
Tor is critical security software with a lot of implementation gotchas. Just like you shouldn't write your own crypto, you shouldn't write your own Tor. Arti is developed by contributors of the original C Tor, I wouldn't go near a Js/Go/whatever implementation from a team with less credentials.