85% of Iowa voters? from what I understand they are late deciders. It’s never wise to assume you have it in the bag, it may cause those first time caucus voters to decide there’s no need to show up. While many of Trump’s supporters see this as sticking it the the media, on the fence voters in Iowa probably have already factored that into their calculations. They may see this move in a different way and use their vote as a against Trump.
Oh, how would you know? You don’t - you just hope. Even if Cruz takes Iowa - that’s the end of the line for him.