Game development trends are cyclical. There was a span of time when co-op features were largely unheard of. Now it seems every other game has a co-op feature (which is a good thing). Similarly, there was a time when many were saying adventure games are dead and now companies like Telltale have made the genre popular again.
The RTS genre may be thinning out now, but I wouldn't be at all surprised if it makes a "comeback" in a few years.