Scala.js has been stable for quite some time (0.6 is effectively 1.0 but author has been holding off on giving the 1.0 blessing). Ecosystem is indeed tiny compared to vanilla JS, but aside from Typescript what alt-js project's ecosystem isn't miniscule?

The value proposition of Scala.js, GHCJS, Bucklescript, etc. are static types, 2-way interop, and all the features of the host language representable in javascript (which is incredible).

Lichess' approach works quite well, I have no issue with jQuery on the client while doing the heavy lifting on the server; not everything needs to be a 20MB SPA.