And not taking the job isn't even the best way to solve that problem. I'd ask them how married they were to CS and if moving off of it was doable.
If the shop is reasonable they won't have any objections so long as you're not hurting delivery targets to pursue your pet project. If they're not reasonable then you've found out a very important fact about that shop and that in itself can be a deal breaker.
I talked myself out of a job offer once when I stated that my main motivation for jumping was to work on a more well-maintained codebase. Kinda glad I did, but it was a wake-up call. If I'd tried to pull that shit when I didn't already have a great job, that would've sucked.
See, that's something I would never do. I dislike your tech stack, and if I can I will stay away from you. Why would I want to ask you to change it?
The tools one uses are one of the countless factors that influence their happiness. Therefore, if I think CoffeeScript and Angular suck balls, I will at least try to stay as far away from them as I possibly can.
It's not a matter of being "so fulfilled in their lives", it's just that I like to work with tools that have a higher chance of making me happier.
It's one thing to try something different in a new project if it can legitimately solve problems we've been having in old projects.
But this idea that you can do that and most "reasonable people" are going to get behind you on it is balderdash. Most reasonable people are going to tell you no because there's not a strong reason to.
Much better to just move on if you don't like the tech they're working in.
And this is coming from someone who dislikes CoffeeScript, and always has. I would never suggest an active project rewrite their CS in something else.