We’ll see what Trump is made of after the IA and NH voters come out. If he loses both of those - maybe he stays in until SC. Maybe.
Winners of Iowa seldom win the nomination.
Ditto with NH. Any one can win. Even Sanders, Huckabee.
Very small electorate (Iowa does not have a primary election, only caucus goers get to vote, usually around 150,000 out of a state population of 3.1 million).
When primaries move to the much more expensive markets such as Florida & SC and super Tuesday, that is when the men will separate the boys.