That doesn't change the fact that D&D is more closely associated with the concept of RPGs, in the mind of the mainstream, than every other pen-and-paper RPG combined. D&D is the gateway by which the majority of gamers enter the hobby.
This is true. But it may be so only because D&D holds this space. If D&D was to perish as a brand, this does not mean that this space could not be filled by other pen and paper rpgs adequately.