It's the "floats all boats" theory and there's a lot of truth to it. As more and more new people find D&D and roleplaying for the first time... many of those people will discover they can play this game in other genres and in other styles and pick up more books across all different brands. Which of course then means those players need to find other players to play with them and thus introduce new folks to those other brands... which will make some of them then find their way over to D&D (and those grows the D&D audience further.)Actually, Paizo says PF2 is doing better than PF1 if you can believe that. All kinds of indie and D&D alternatives are doing very well right now. Of course, they pale in comparison to D&D and always have, but they are seeing a lot of desire for products. What it tells me is that a lot of folks playing 5E are likely also playing other games in addition to, as opposed to instead of.
It is never a bad thing when multiple types of a thing become a thing. Because it just means word of mouth to those outside the circle just expands and expands, and those who are on the outside might find reason to move in. And that makes all the things bigger.