The absolute worst case for the Republicans is if a late entrant runs, and he turns out to be another Rick Perry and collapses. That would be a embarrasment to the establishment, reinforce the perception that nobody likes any of the Republican candidates, and also increase the odds of a borkered convention.
This. Remember when Perry got in (himself a late entrant), and everyone talked about what a skilful politician and great debater he was?
Running for president is completely different from running for governor, even in a big state like Texas or Florida, and these days it's pretty easy to crash and burn in a matter of days if you're not ready for it.