Trump's not going to be affected by this. His entire campaign is masterfully constructed in that he can do basically anything and justify it on some kind of loosely internally consistent basis. If he doesn't show up, it's because he knows Fox News and Megyn Kelly are just going to smear him, and he's not going to bump their ratings only for them to treat him like shit, and don't we want a president who'll refuse to deal with opportunistic assholes like Iran? If he gets Fox to take Kelly off the debate and agrees to show up, then he proved what a great negotiator he is, and oh, don't we want someone that crafty dealing with Russia?
Say what you will of Trump, but the reason he's done so well is that he's perfectly in-tune with what his potential voters think of him and manipulates it absolutely perfectly. I don't see any reason to think he has erred in this case.