I think it's inevitable that something other than JS will be adopted eventually, but we could be talking quite a lot of years before it's natively supported in all the browsers you need to support. Which is the same as saying: we'll be transpiling to JS for the forseeable future.
Google made a play with Dart a few years ago; but for a variety of reasons the developer reception to Dart was cool at best. They cancelled the idea of supporting Dart natively in Chrome and focused on transpiling to JS.
Web Assembly is really interesting in that it's not aimed at being a broadly-written language, so much as a powerful/high-performance transpilation target (github.com/WebAssembly/design/blob/master/HighLev…). Plus it has representatives from the major browsers and so has a reasonable chance as far as web standards ever go.
So when you take stock:
So basically it's transpilers all the way down, but we should eventually have a target language that runs as fast as the browser's own code.