I'll engage on this because I think it's an interesting question.
I (with no evidence) suspect that if D&D never existed, and some other reasonable trad RPG like MERP, Runequest, or WFRP had been the first published RPG instead, with the same level of marketing and support, that game would be (and have remained) the market leader like D&D is now.
Maybe not by as much. I don't deny that Gygax D&D has a certain kind of magic in the text and the imagery. Equally, I think some of its assumptions also put some people off, so maybe a MERP as the first ever RPG would have done even better. Who knows.
I suspect that if the first RPG ever had been something less trad, like FATE, PbtA, or Vampire, it could also have become market leader in the same way, albeit the original grassroots and playtest culture couldn't have come from the pre-existing wargaming culture, and would have to have built from something else (improv theatre?).