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.
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.