That's not exactly a new phenomenon though. Every new D&D edition sucks up the oxygen for a while until that edition starts to stale and the new folks brought in by all that marketing start to discover other games. Then they increment the E counter again and we do the dance all over.

That’s also a factor but I usually talk about D&D as the plural D&Ds, the cultural pull that makes all RPGs be some version of D&D. Doesn’t matter if it’s a new edition, whatever Paizo is doing or OSR vs NSR. Fit under that umbrella, otherwise you don’t exist as a #ttrpg