Don't be too sure. If you look at the polls from 2012, after Santorum beat Romney in Iowa, Romney dropped and Santorum jumped - narrowed the gap between them by 12 points in a week. Romney still won, and I think Trump will still win. But if both Rubio and Cruz get a bump from beating expectations in Iowa, and Trump loses some support for doing worse than expected, it could wind up being another close 3-way race in NH. We will have to wit until about Friday to see how the polls are trending, but I think we will see a lot of volatility in the next week in NH.
It may change by a few points but NH is not going for Cruz or Rubio. Iowa is a short bump. Later as others drop out the polls will begin to tighten.
Also remember that was with Romney getting most of the publicity for having ‘won’ Iowa. Santorum’s win was recognized way late. Good vibes for Cruz and Rubio and bad vibes for Trump are going to get full press coverage this cycle so there may be significant effects on NH.