The biggest threat to Trump not getting the nomination RIGHT NOW is the conspiracy underway by millionaire elitists and Washington Establishment GOP to steal the nomination at a brokered convention.
Rubio has no shot in hell at beating Trump, but he is staying in to ensure he sucks delegates away and that there is a brokered convention. I would not doubt that the GOP Establishment have not offered Romney and Rubio (not necessarily in that order) the ticket if they can make it happen. The GOP Establishment hates Trump, and they hate Cruz.....Kasich might be in the mix, but I tend to believe the Establishment would rather see a Romney/Cruz ticket.