The Rails MVC architecture is already past its prime, since it solves the problems of 2004. It's possible that startups may move more toward backend-as-a-service + JavaScript front-end instead of using Rails as their MVP.

In any event, the transition should be slow and I fully expect Ruby to be around for a long time, but people may notice a slow decline in demand for Rails programmers in the next few years compared to other hotter technologies. Sinatra (or some other Ruby framework) may replace Rails as a backend.

I'm a Rails programmer and I don't bristle at the suggestion that market demand changes. I welcome the change and adapt. I don't mind learning new languages.