Since caucuses are so diametrically different from primaries, in the way that a candidate ultimately has to garner support even from voters who don't have him or her as their first choice, it just makes the chance of Trump actually winning Iowa infinitely much smaller.
You're thinking of the Democratic caucus method, where your candidate needs 15% in that precinct, or you have to pick someone else. The Republicans don't do that. They just have a straight up straw vote, and you're free to vote for a candidate who no one else supports, and it still counts.
