I don't think the CoffeeScript advocates did anything wrong at all. CoffeeScript was a popular solution with a healthy ecosystem, no one knew when JS was going

I don't think the CoffeeScript advocates did anything wrong at all. CoffeeScript was a popular solution with a healthy ecosystem, no one knew when JS was going to get these features, and TypeScript was still an emerging player (just like other typed JS variants like Flow). Also, when I wrote in CoffeeScript, (1) I loved the JavaScript ecosystem more, (2) I understood the code I wrote better and built and learned more quickly, and (3) I understood the nuances of an JS-extension language, which helped me quickly understand what TypeScript was when I heard of it. I'd assume that other developers got some of these benefits as well, and these benefits may have been well worth the cost of converting from CoffeeScript later.