Bethesda has a massive audience that has grown progressively over the past decade thanks to Oblivion, Fallout 3, New Vegas, and Skyrim. At this point, anything they release is all but guaranteed to be a smash hit. Ditto for Bioware; regardless if they were to release a new Dragon Age or Mass Effect, you can bet your left ass cheek it'll sell millions of units before the door is even open.
Fantasy games are hardly taking over the market; they've always held a considerable niceh. Nor is fantasy taking over the RPG genre; it's always been the prominent setting for both electronic and tabletop RPGs. Shooter RPGs, on the other hand, are a relatively more recent phenomenon, which is in part why they're not quite as common. But they can still sell remarkably well. The only reason we haven't seen them outselling the fantasy RPG is because, well...the most prominent RPG developers have been busy pushing out their fantasy RPGs, as we've seen with Skyrim, Dragon Age: Inquisition, and The Witcher III. It's not hard to outsell something that doesn't exist. But that's going to change soon; Bethesda will likely release the next Fallout, while CDPR will start working on Cyberpunk 2077. And when they do, some people will then start to say that fantasy RPGs are going out of style. I've seen it happen back when FO3 and ME were in production, and those claims were just as laughable then.