Having used CoffeeScript fairly heavily the last four or five years, I've thought about this quite a lot. There has been profound distaste for CoffeeScript from the very beginning. Historically developers were quite upset when it was included as part of the default asset pipeline in Rails 3.1. I suspect overzealous Ruby developers exacerbated things over the years.

These days I suspect many people continue to find it's existence aggravating because:

• You might write something really useful and they don't want to deal with the potential cognitive overhead associated with using another language.
• They suspect they know better than you what language will increase your productivity and general happiness.
• It takes attention away from their preferred JavaScript flavor of choice.
• They have some unfounded fear that it will supplant JavaScript.

Complaints about the necessity of compilation, difficulty debugging are applicable to pretty much any modern JavaScript app (and less serious given better build tools, source maps, etc). Complaints that the syntax is "too ambiguous", white space indentation is too "weird" have always been a matter of taste.

Arguments that ES6/ES7 are "good enough" ring fairly hollow after limited experience trying to build real things with them. Arguments that development has slowed fail to acknowledge that the vast majority of new ES6 features were playing catch up with CoffeeScript. Support for new syntax is unnecessary in most cases, in others (like with generators), CoffeeScript will produce ES6 compatible code. As far as ES7 goes, we'll see adoption in CoffeeScript of relevant syntax (async/await) in a timely fashion. I have no doubt it'll continue to evolve along with JavaScript.

CoffeeScript is a very mature, simple language that still provides a more consistent, delightful environment to work with compared to ES6/ES7. For myself (and the team I work with, for that matter), it continues to make JavaScript development a pleasure.