Better read up on George Will today as he floated that if no conservative is nominee, a 3rd party bid will be made.
So, only Trump had to sign a pledge not to run third party?
In almost every state there is a “sore losers law”. Look it up. It says that after the first state primary (Iowa) anyone who was on the ballot is ineligible for a third party run. A third party run will need to be fresh blood or someone like Perry who dropped out before the first primary vote. Trump lost nothing by his agreement if he was not going to ditch the GOP before Iowa.