Trump will fade once the field is narrowed to 4 or less.
What do you based that on? I mean other than wishful thinking?
Okay, number time.
RealClearPolitics - Election 2016 - 2016 Republican Presidential Nomination
You basically have three paths.
Trump is at 29.5 in the RCP Average, off his high of 35%, but his win in NH might boost him again. His support has not gone below 25% since November when the crazies supported Uncle Tom Carson for a week so they could claim they aren't racist later.
You have the Establishment lane- Which is (or was) Rubio, Bush, Kasich, Christie and MAYBE Fiorina. Combined they have 31.1% of the vote. Rubio has 17.8% of that, but after he shit himself in the debate, expect that to start drifting over to Bush and Kasich. The thing is, the Establishment hasn't settled on a candidate and they won't anytime soon.
Then you have the Teabagger Lane, which is (taking out Trump's 30%) Crazy Ted and Uncle Tom Carson. Combined they have 28.8. I think WHEN Carson comes in fourth or fifth in SC, he'll finally figured he's sold enough books to White People telling them how shiftless all the other Negroes are and can go home. But it's just as likely his followers will go to Trump than Cruz.
So effectively, you'll have a three way race between Trump at 30, Cruz at 30 and the Establishment guy (whoever they can get somewhat enthusiastic about) at 30.
I don't see Trump's support going to the Establishment guy. It MIGHT go to Cruz, but then again, the more people see of Cruz, the less they like him on a personal level. So maybe not so much.