I'm not sure if getting some extra syntactic sugar is worth adopting a whole other language into a codebase: at least, that seemed to be one of the lessons from CoffeeScript.
var that = this;
function() {
...
}
CoffeeScript didn't die. JavaScript (ES3/5) died and we are all using CoffeeScript now!
That being said, as much as I liked using CoffeeScript in my own personal projects, adopting it for a non-trivial project at work probably turned out to be a net mistake all things considered. The only positive was that the .js output files where clean any easy enough to work with that we could quite easily just drop CoffeeScript and continue developing directly with those .js file
If it ever becomes a liability, you just check in the "transformed" code and it's gone.
That said, I always found CoffeeScript to be a worse language (syntactically) than JS.
I've done this and regretted it - CS transpires to ES3 and specifically null chaining is completely unreadable.
From a backend perspective it's no biggie to instead keep the CS dependency and gradually convert files manually when you have to make changes anyway, or when you have 15 minutes to spare between meetings.
You become fluent enough to not even need tests after a while (yikes!).