Many of the great features of modern js (destructuring, fat arrows, splats) came directly from CoffeeScript. As much as I loved the comparative power of CoffeeScript (list comprehensions, everything is an expression), its time has past. There is very little active development and it has lost all momentum. It also never started addressing the biggest issue facing large JS projects, typing and static analysis. Refactoring a CS project is a fragile process that I won't miss.
Are you sure CoffeeScript has lost momentum? It now supports ES2015 imports and exports, and I've seen quite a bit of work on the 2.0 branch (and various forks where contributors are adding new work before merging back to 2.0) to add support for extending ES2015 classes. It looks like that PR will be merged soon.
Also, how much momentum does it need? The language is complete and usable as it is.
I write this as someone who does lots of professional work in ES2015, and very little in CoffeeScript. But every time I come across CoffeeScript in a project, I find myself admiring how nice it looks, and realizing how ugly ES2015 looks in comparison.
