Eventually there will be “Trump fatigue”, but the other candidates aren’t so hot.
Right now, he would wrap up the delegates by April. He will win NH, IA, NV caucus, SC, NC, GA, FL, TX, AZ, MO, AL, and OR. He leads most of these by double-digits.
He has more money than any GOP candidate in history, and is only getting more by the day. He spends virtually nothing. His "war chest" is four or five times that of Hillary Clinton (who will be indicted or under such a legal cloud she can't win).
Trump has no need or reason to go third party, even if, as expected, the GOPe abandon him. He won't need them. It will be a joy to watch.