Other
Looking at my shelves, if the d20 D&D game disappeared, I would be left with Star Wars (which is kind of a cheat of an answer since it really is d20). Sometimes I contemplate quitting everything else for SWRPG anyway. It might be the only way I ever get to play it again.
My next option is AD&D (which is alos a kind of a cheat of an answer since AD&D is really D&D). But I have a lot of good, old material for that game; and I find it quite enjoyable as a core game.
My next option would be SW Minis with the new campaign book (but see first paragraph cheat).
So, I guess I'm left with ORK! It's a simple system, but it doesn't last too long in play. Otherwise, Villians & Vigilantes is on deck; but it probably wouldn't last too long either.
I don't think I could return to Rifts or Heros Unlimited. I would play Rifts, but I can't run it.
Without D&D or d20 derivatives, I am proibably done with RPGs.