If he does, understand that 1) POTUS left office with 95+% approval among republicans. 2) POTUS took a big chunk of black and hispanic votes. 3) POTUS has the backing of many dems / former dems, especially the blue collar types.
If POTUS starts a third party, it will NOT just "split the GOP"... there are plenty of voters who dems target that will be peeled away too. If they could be moved to vote fora republican, they can certainly be moved to vote for a non-GOP party.
Better to split the GOP vote in half than allow a uniparty charade to continue. There's really no difference between a 51-49 Dem-GOP split and a 100-0 Dem-GOP split.
Also don't forget that the reason Trump's work can be so easily undone is that he had to do everything himself through executive action. The GOP never helped. Once.
They will never help the common people or investigate voter fraud (they love to lose like McSally in AZ 2018) unless forced by a third party. It's time for the GOP to die or join the Dems, we don't have use for the traitorous fucks anyway