"Let's have nineteen people spend a YEAR beating the shit out of each other, and specifically out of our eventual nominee, seeding resentments, misleading memes, explicitly claiming our eventual nominee is no better than Trump. Then we'll really know our candidate is tested and ready!"

We had a ten-way primary in 2004. I think that's the biggest one in the modern age (until this one blows it out of the water). We ended up with exactly the candidate everyone expected from the beginning, only he was sorely weakened by attacks he'd received from the left (which depressed base turnout) and no more prepared for the attacks he'd receive from the right. And Bush, who nobody imagined could possibly be reelected, finally won even the popular vote.

The exception to this kind of scenario is the nightmare that Reeq seems to be imagining -- the Donald Trump scenario, where some idiot outsider candidate catches on, badly loses the general (Bernie, McGovern), or if not, is woefully unprepared for the job (Trump).

Big primaries might SEEM more just or democratic. (They really aren't; 20 candidates out of 100 million Democrats is really no better than 3 or 4.) But they certainly don't make it easier for the party to win.