Tech is an ecosystem, you can make just as much money off dead trees as you can off of killing wild boars. Coders who can work on both legacy and brand-new applications are the most versatile. If you think your skills are being neglected just because you're on Rails instead of Node or Elixir, or CoffeeScript instead of ES7, you've got a hard career ahead of you.
The lesson isn't to migrate every year to the new hotness, it's to pick proven tech that is going to still exist in ten years. The Lindy effect applies.
Fewer technologies, with better support and bigger communities, are better than a fragmented ecosystem.
Fringe technologies come and go. The only reason to bet on a fringe technology is if you see major advantages, and in that case it is unlikely to remain fringe. Once it has been around for a few years and adoption is flat or dropping, yes, it is time to move on. That's just the cost of making a bad bet.
The number of technologies you can learn in your career and really master is limited. Too many people forget that.
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.
And no: unlike someone else suggested down here, I won't suggest you move away from a technology just because I don't like it. If you like it, keep it. If it's needed, I will adapt like I always did.
Just to put things into perspective, I recently had to do maintenance on an accounting app written in Visual FoxPro, which is another cobbled up piece of absolute shit. I hate it, and yet I did it. I can do and did the same with CoffeeScript, but if I can help it I will gladly avoid it.