Yes I think it's a fantasy too. Anther possible fantasy is Trump not going third party if Cruz wins the nomination.
The Trump fans seem to be taking a page out of the playbook of odious RINOs like Mike Castle, Arlen Specter, Dede Scozzafava, Lisa Murkowski, etc., etc.
It's ALWAYS a one way street with them -- they will endlessly bully conservatives and DEMAND conservative voters pledge to support them in November "or else the Democrats will win", but the reverse is NOT true if they lose. If the RINO loses the primary, they NEVER promise to "get behind" another conservative. They will happily endorse the Dem out of spite or bolt to a third party to "teach the GOP a lesson" of daring to nominate someone else.
I see the same tactics with Trump fans. Endlessly, they demand conservative voters who dislike Trump still pledge to support him if he's the nominee -- "Trump or Hillary, pick one" But, of course, none of them demand Trump pledge to support another Republican if HE loses the primary. Trump was noticeably the ONLY Republican on stage to decline to raise his hand when asked to pledge if he would support the Republican nominee in November, and his fan club APPLAUDED him for his "independence" and "courage" to "stand up to the GOP establishment"