Harris will win handily because Republicans never built up a platform and instead went all in on tying Hunter Biden to corruption and portraying Biden as old. All of that is now out the window, and Harris will get a honeymoon boost that alternative candidates initially receive due to being "different," much like Trump got in 2016. Harris will come in as a mixed minority woman who is sharp and doesn't suffer fools. She just has to rally the Democratic base to show up at the polls to vote, and they win.
Trump will continue to get smeared by Epstein and various other issues that make him look like a total asshole, which won't be hard because he is one. There isn't anything that the country is reaching for as an alternative to him anymore because he messed up his first term with grift and incompetence. No one believes he can fix anything; instead, it's just "hate on libs and put in more conservative judges."
I strongly suspect that the decision to swap out candidates was made a while back before the debates, and Joe, with his mouth open during the debate, was set up for that. I think the decision was maybe made a year or two ago, and Biden didn't want to run again. Remember that Biden did not initially want to run for president and was prodded into it. He is now older, and both Democrats and Republicans have been making a big deal about either candidate's age — "They are both way too old." Instead of the DNC trying to make Biden look good, party members were saying the same thing. I think when they did that, they knew the swap might happen and knew Trump would have too large of an ego to ever step down to avoid questions about his age. This trap might have even been set with "women, man, camera, TV" in that question. Trump's age or condition would, by his nature, force him to attack Biden as being "old or senile" as an act of deflection, which wouldn't work against a younger candidate. I think Trump got boxed into a bad position by the DNC pivoting around him and knowing how he would react to being questioned as to being "too dumb" and "too old". Now Trump is up against a candidate that is younger, and smarter than him by miles and only will make him look "very old and senile." Trump has been throwing "old and senile" stones while living in a glass house for the last eight years. Trump would never step down for a better GOP candidate so everything used against the Trump and ultimately the GOP is still in play.