I sincerely hope Scala.js isn't "dead". Though I know use by academics isn't the use case you imagined, Scala.js has very much been the right choice for me publishing interactive Open Educational Resources that can include all sorts of little simulations, models, and programmable games. It's not the sort of thing I'd have time to do in any other language, but via Scala.js (and a front end framework I wrote) I can put together something that (though it's scrappy because I rarely get time to add the polish) works well enough very fast.