Youre saying that RE4 remake sold the same number of copies as Anthem. So either Resident Evil, a franchise that is basically a household name since the 4th game with multiple movies, is not mainstream, or Anthem is? Resident Evil is equally as mainstream as Mario or Minecraft is, because becoming mainstream does not only depend on sales.
If someone who didnt play games saw you playing a certain game, would they think you were a nerd for playing that game? Generally, that is a good metric to measure what is mainstream or what isnt. People will look at a game like Stellaris and instantly think only a nerd would play it. And I mean, yeah, only a nerd would play Stellaris. But they wouldn't think that only a nerd would play Resident Evil, and they would probably be able to guess it was Resident Evil, despite themselves not playing video games.
Regardless, my point still remains that I agree with the CEO of Crate. RTS cannot be a mainstream genre without losing its core identity that makes the RTS genre different from other games.