What would happen if Duke Nukem had failed his mission?

GamesRadar - Paradise slipped through our fingers--and it’s all 3D Realms’ fault. Well, not really, but it’s fun to think about. In 1996, Duke Nukem saved the world in his first 3D excursion, and a macho, loud-mouthed franchise was solidified in the FPS continuum. But what if Duke Nukem had failed miserably in his struggle against the aliens? What would’ve become of Earth as we now know it?