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...