The reason we believe that humans wiped out the mega-fauna is because the megafauna of North America all disappeared within a millennia of humans arriving from the land-bridge. It's possible they all coincidentally disappeared but unlikely.
I don't consider it unlikely at all considering that the human entrance into N. America coincides with huge climate change events - the Younger Dryas cold snap followed by the abrupt and quick Holocene warming event. It's even possible that the human migration was a result of looking for favorable conditions as climate change made their traditional hunting and foraging areas untenable.