That was very likely the case early on with WoW. But once you got past the first expansion I'm pretty sure WoW had more subscriptions than all previous MMO's historically had combined. WoW ended up being the first MMO for a very large segment of it's player base I would wager.
I think that most people would identify themselves as a fan of a specific genre before all others. But that Genre is likely to change on a regular basis for most people. Some people do seem to always play the same genre games and rarely venture out of it. But most gamers that I know would change that favorite genre whenever their latest game of choice switches.
Personally I would say that in the last half dozen years my favorite genre has been sandboxes. I've played a lot of Minecraft, Terraria, and 7DtD, which are all sandboxes in the strictest sense. I also played a lot of Skyrim which isn't exactly a sandbox but is very open world. Before all that though I played WoW, and that alone, for several years. I played SWG and EQ before WoW came along. And I had some bouts with Diablo 2 as well. I play other games here and there as I'm drawn to them, D3 gets my attention for awhile after every major patch, and I actually completed the Shadowrun game. I expect in a few years though I'll have moved on to some other genre and claim that as my favorite for the time being.