There are five good reasons why Donald Trump should not quit the race.
The first is technical. Most states use paper ballots, either they are counted by hand or fed into scanners. Those ballots have been printed. All states uses paper ballots for absentee voting. Absentee ballots are already being distributed (and have been for weeks).
The second is a matter of electoral fairness. For both absentee voters and voters in states that allow early voting, the election is, in fact, under way. They've made their choices based on who is on the ballot. It would be an unprecedented bait-and-switch for Trump to drop out.
The third is notice to the voters of who are the candidates. If Trump drops out, then another vice-presidential candidate would have to be chosen. There is no mechanism for that, and likely, the choice wouldn't be made until after the election. The voters have the right to evaluate the candidates and make their choices; Trump dropping out would say to the voters "trust us, we're going to pick a decent man."
And trust whom, exactly? The Republican party elites? Their views were soundly rejected by the party's voters in the primary contest. If the opinions of the party elites mattered to the GOP base, the number of Trump voters would've not filled up a short bus.
Which brings me to reason #4: Trump received over 13 million votes in the primaries, which were the most ever for a GOP candidate.
The "will of the people" is one of those concepts that politicians speak of in glowing terms and then disregard at the drop of a hat. Trump's flaws were well-known to anyone who had bothered to look and yet, the GOP voters chose him. They turned their collective backs on the party elites and the professional pols and chose Trump, a completely untested amateur, as their guy. You can bet that most of those voters will look on a Trump resignation as his having been pushed out by the party, a penultimate backstabbing by the pros.
And then to tell those thirteen million Trump voters that they're supposed to turn out for a clown that wasn't even running in the primaries? Good luck with that.
The last is Donald Trump. While he might be able to accept being defeated as either evidence that he couldn't close the deal or console himself with conspiracy theories about how Hillary Clinton conned the voters or whatnot, there's no such consolation to be had should he drop out. Dropping out is Donald Trump having to acknowledge that he's a loser. He would have to essentially fire himself. He would be a self-admitted failure. That would be a reversal of his entire personality.
Besides, all you have to do is look at the Right's reaction, which is "Bill Clinton was worse". Expect to hear a lot of that from The Donald.