Coffeescript was 'replaced' by ECMAScript 2015. Ember is fine but it's not the end of the line - it's been superseded' in most scenarios by React and Vue.js because those are generally better tools.

JS technology changes quickly not for its own sake, but because there are still discoveries to be made about how to improve both syntax and how we think about code. I'm sure the pace will taper off sooner or later, until the Web is taken over by something else.