About 80 percent of the electorate are low-information voters. Many of those are attracted to sound-byte machines like Trump and sparkly paint on bass boats.
I'll give it to Trump, he's found the loophole and he's gaming the system. He's got a rock solid 30% of the GOP vote (which means there's 70% that he doesn't), but the remaining 70% is divided amongst "real candidates" who are bickering and clawing like a pail full of crabs. If the people that have zero chance of getting the nomination would get out (I'm looking at you Carson) there might be a chance at preventing him from getting the nomination. If Carson/Kasich/et.al. stay bitterly in the race, they will wind up handing the nomination to Trump. It speaks to a larger issue of a leadership void at the RNC.
I have ZERO doubt that if Trump wins the nomination a third party candidate (Rick Perry most probably) would have legs as a "Conservative" alternative. The theory behind that is it would keep conservative voters from sitting home and killing the down-ticket Republicans. Perry, Romney, Gingrich...there are all kinds of possibilities there. Thank God Palin already endorsed Trump or else she might have given it a thought. The downside of course is that it means at least 4 years of Hillary and a tilt of the Supreme Court with 5 hard core liberal justices.
A brokered convention is also a possibility, but then you run the risk of pissing off the Trump voters (and they would have a legitimate beef).
The final possibility is that the GOP embraces Trump and he actually freaking wins the nomination (and potentially the Presidency). Hillary is a terrible candidate and is beatable. But that's a huge risk given Trump's insanely high negative poll numbers. There are a lot of people that don't like/trust Hillary, but there is a large swath of the population that HATES Trump and would be energized to vote against him.