I don't think it's really that MMOs are becoming less popular, but I do think their growth has slowed significantly. The genre is growing stagnant. We are lucky if brand new MMOs today are as good as WoW when it was released a decade ago, and almost no one is trying to things in the genre.
I agree but there are a couple of upcoming MMOs that do things at least a little bit different. Even if "snadbox" seems to be the new trend word, it's refreshing to see more of those mechanics in the upcoming games.