There are no Non-Trump Republicans.With all the baggage he will be carrying by that time, (assuming he is not in jail or dead) I don't see it being a cakewalk in the primary. A decent challenger could eviscerate him on stage and beat down any hopes he had of garnering Non-Trump Republicans /Independent voters which would make winning a primary that much more difficult.
Not to mention his pocketbook will likely be much lighter after all those loans come due and he uses the current contributions to pay that debt.
I don't see him running because of a multitude of factors, but if he did, a primary win is not a foregone conclusion.