Trump wont say he wont run as an independant If he doesnt get the Rep nomination. If hes a true Republican and truelly believes the Democrats in power are "stupid" why do something he knows is going to hand the election to Clinton and keep the stupid Democrats in power? Ross Perot part two? Has an independant ever won the election?
Republicans know he can't win a general and that he is causing a lot of disruption in their primary right now. The party bosses want to get rid of him. Fox News which is the defacto media outlet for the Republcian Party wants to get rid of him which is why Magyn ripped into him right from the getgo. Trump knows it so the threat of a 3rd party candidacy is the only thing that keeps the wolves at bay so to speak, as far as he is concerned. In reality I don't think Trump would go for a 3rd party run, but right now he has to keep it on the table and the Pubs can't really afford to call his bluff. IOW he's the 800 pound gorilla in the room and no one dares mess with him.