The perception of legitimacy is important. A political party is a voluntary association, and Trump's voters have no obligation to stay Republican or support Republican candidates. If he is seen as having lost because of rigged rules, anti-Trump Republicans will win a Pyrrhic victory, as many of his backers will walk away in the general election.
Amazing! Someone admits that Whitey has no obligation to vote Republican! Stop the presses! There is no inherent law of the universe that binds White voters to the GOP!