So now we step into a newer area. Candidates are dropping like flies (well after the fact), but Jeb Bush is still in it. And you know what, I think Jeb Bush could still be the nominee (some time in the writing of this post, Jeb will drop out now

).
Sometimes some things are easier to see in hindsight, but I'll give you foresighted hindsight. Trump can't debate. Yeah sure, he can talk, but he can't debate. And he hasn't had to debate yet, just say a few things and smirk. That stuff works with a "debate" including 10 candidates. "Make America Great Again" works in stump speeches and when you talk about 15 minutes in a 2 hour debate. But what about for 1 hour in a 2 hour debate? As the field has tightened up, vague talk becomes more pronounced. Where Rubio failed in the crap debates, Trump most certainly can fail in a political debate (what is called one these days).
Trump has a lot of support based solely on his personality. A bad debate performance could tear right through that. His base could remain, but the voters that put him to 30 or 40% may waiver and look for alternatives. That would be Cruz, Kasich, or yes Bush. I think Bush has one thing going for him, Super PACs that have invested way too much money into him to quit. So the goal right now is to wait long enough for Trump to burn out and look like a fool in a three or four man debate. That will tighten the race and then all of a sudden the Establishment is one on one with Cruz.
May not happen that way, but I think a tighter debate is not in Trump's best interest and Trump doesn't know that yet. I think he has possibly seduced himself into thinking he is an Obama star like candidate, when he isn't even close.