Throwing in your lot with a single company?

I currently work for a company that I like. I like the people I work with, our domain is very unique and interesting, there are lots of interesting challenges to solve and I have a great work-life balance. It's small, but it's been around for a couple of decades so it's unlikely to go under.

Here's the problem: it's very unique. The domain itself is something almost nobody else does, for various reasons, and the product is even written in an in-house language. We don't use many industry-standard technologies or practices.

It feels like every year I stay here, I'm becoming more valuable to this company but stagnating in terms of the industry at large. My general programming skills are certainly growing, but it's hard to put that on a resume. On the other hand, I was told at my last performance review that I'm seen as a rising leader in the company.

Is it terribly risky to stay here for the foreseeable future? It's been 3 years now; I'm starting to get a bit anxious about falling behind on industry standards. I do some side projects to keep in the loop, but there's something to be said for real-world experience. Should I move on before it's too late? Or am I worrying about nothing?