That may very happen.
Trump will do this debate, then declare it a fiasco, which it might turn out to be.
He then announces that he knows how to run these kinds of things much better than FOX does.
That statement, of course, will be true.
Then, Trump arranges a new, and better, debate.
His setup then becomes the norm for many elections to come.
No high-profile GOP candidate is going to defy RNC rules and participate in a non-sanctioned debate. Any president is only going to be around four or eight years max but careers have to go on.