I think Kos lays it out well: a Trump 3rd party run is actually a boon for the Republicans:
Another side effect: it would be very clear to Trump that the national party was conspiring against him, and he could start gathering petitions for an independent bid. But would that be so bad for the national GOP? Remember, at this point they’ve already surrendered hopes of winning the White House. Their panic is more about protecting their down-ballot candidates. Having Trump take his marbles and go independent would be a positive for them, particularly since those Trump voters would still vote for the rest of the Republican ticket while giving the GOP the ability to distance themselves from him.
To the point: Trump wins the nomination, moderate Republicans stay home. Trump loses and exits quietly, loonie Republicans and whack-job independents stay home. Both Trump and Jeb!/Rubio/whoever run, they lose the White House (a given under any circumstance) but they have all their voters out, and maybe even more if the schism is particularly vicious, and the downticket Republicans get it all.
I was just thinking about the Goldilocks effect. If Trump is out there then the Republican nominee with simply look more reasonable than they otherwise would. Pundits might say things like, “Well, the fringe of the party has borken off to support Trump, so [Republican candidate] really represents the values of the sensible, fiscally Conservative Republican party of yesteryear.”
In other words, if the Democrats are the $0.25 bottle of shit ,and the Republicans are the $0.75 bottle of shit, adding a $101 bottle of shit to the menu might make some people decide to splurge and vote Republican.
Note: Here ‘$’ is a symbol representing your soul, with any excess beyond your own soul to be paid in the souls of orphans.