So, yes, we would likely have other RPGs without D&D. They just might look different than what we currently call "table-top role playing games". Maybe they'd come out of the parlor-game/murder-mystery game root. Or maybe someone would come at it from the classic boardgame root - Monopoly, but running around Middle-Earth. Or maybe SCAdians would have come up with an option for their form of play that didn't require hitting each other with sticks. Or maybe someone in Japan would develop a game in which people took roles from anime...