The only flaw in this argument is that Donald Trump actually won because Republican voters voted for him more than they did any of his opponents. He won because Republicans wanted Trump to represent their party.
It's true that Trump did better in open primaries and caucuses, ones where non-Republicans could vote. He won 18 open contests and 13 closed ones. He lost 12 closed contests and lost five open ones. But that overlaps with the split between primaries and caucuses. He won 26 primaries and lost 11 caucuses, according to Real Clear Politics' definitions. He won 81 percent of the open primaries and 82 percent of the closed ones. But he won only half of the open caucuses and about a quarter of the closed ones. It was primaries where Trump thrived, not open contests. Primaries -- where more people vote.
Exit poll data shows that Trump didn't get a disproportionate amount of those votes from non-Republicans. Exit and entrance polls ask voters to identify their partisan leaning, meaning that what voters say doesn't necessarily line up with their party registration. On the Democratic side, there's a valid question whether or not the strong support from independents for Bernie Sanders -- more than a third of his support in contests for which we have exit polls has come from non-Democrats -- follows candidate choice or leads it. That is to say, are Sanders fans saying they're independents because they like Sanders -- a longtime independent himself -- or are they non-Democrats coming to vote for his candidacy? (The former clearly happens, but the percent of the electorate that identifies as independent is only up slightly since 2008.)
That's not what's happening with Republicans. Trump consistently sees about the same amount of support from independents as Republicans -- unlike Sanders, where there's a gap. (Lighter colored bars indicate states Trump lost.)
But as a percentage of his support, most of his support comes from Republicans.
A third of Sanders's votes are from people who say they're independent, on average. For Trump, the average is 10 percent -- much lower than the average percentage of independents in the electorate.
There are five states where Sanders's support from independents handed him a win. That is to say, there are five states where Sanders lost with Democrats by won so handily with independents that he won the state.
There is actually only one state for which we have data that meets that definition for Trump: Missouri. In Missouri, Trump eked out a win by a tiny fraction of votes cast, but exit polls showed Ted Cruz winning among Republicans -- by a few points, well within the margin of error. In every other state, Trump lost if he lost with Republicans and won if he won with them.
The reason Donald Trump is the Republican nominee is not that he was brought by the enemy to the gates of the establishment in disguise. It's that people on the streets of Troy opened the gate wide and welcomed Trump in, because they wanted him to be their leader.
Fixing the primary rules to ban big wooden horses isn't going to fix that problem.