If D&D went away....
I'm currently running two adventuring parties using the D20 Basic Fantasy Role-Playing Game, with bits from other retro-clones, AD&D1 Oriental Adventures, and FATE. My groups' second or third choice for traditional fantasy would likely be Pathfinder. I have dozens of sourcebooks and modules for past editions and other systems, so these work best for my homebrew sandbox settings.
Since the advent of D&D4e, which many of my gamers found to be too tactical and not as flavorful as past editions (but YMMV), we've been quite happy playing everything from FATE to Savage Worlds, so if the brand somehow went away or D&D5e/Next doesn't capture our imagination and attention, we'll be just fine.