It's really not about him losing votes, though. Despite the fact that there are a truly mind-boggling number of idiot cultists out there who are for some reason devoted to this moron, there are not enough of them to win an election. Both candidates need to appeal to the vast pool of undecided or apathetic voters in order to win. In 2020, the Democrats won because that pool of people were highly motivated to oust Trump after living through 4 years of his presidency, but people have very short memories and even shorter attention spans, and so the Dems can't really count on the same thing happening again in 2024.
When Trump was running against Biden at the beginning of this race, things were looking grim for the Democrats, because that group of morons who can't be bothered to pay the slightest bit of attention to what's happening in their government (apathetic & undecided voters) decided that they couldn't really tell the difference between two senile, doddering old men, so Trump's rambling incoherence wasn't that big of a problem. Now that he's facing someone more than 2 decades younger than him who still has the complete use of her mental faculties, it's imperative for him to do SOMETHING to make himself not look like a shoeless old man in a hospital gown wandering between lanes of traffic after escaping from his rest home. This debate didn't do that.
It looks like we have an answer to the question "what happens when the FBI shuts off the Russian money pipeline to American right-wing grifters?". Apparently, the answer is that they organize a griftcon.