There were polls showing Romney winning this far out too.
as for turnout: Primary Turnout Means Nothing For The General Election | FiveThirtyEight
The left destroyed Romney, and the social media meme game helped. Trump is better there than Romney, but much worse when it comes to actual experience and tact.
Is it possible the world has changed enough for a populous candidate with negatives comparable to David Duke to win? Sure. Do I think it has? Nah.