I'm curious about the fate of Rails(and other frameworks like Django) in a few years, seeing that many web applications are moving to the Single-Page-Application model and the server simply acts as a rest backend. Perhaps there will still be a large demand for sites that aren't SPA applications. Interesting times.
Single page is an option for some people, and there are several ways to get you there as well. But that's the great thing about the web: we have all kinds of ways to deliver an end product.
That doesn't make other rendering methods go away, though. Flash is still around, for example. And there's really no reason to project that rails will suffer some terrible fate in a few years time. It's been around for several years now and has done nothing but grow, so as long as it solves a problem and people go to it, it and other frameworks will hang around.