More than four years ago, I wrote a diary here about Biden's clear cognitive decline. I was far from the only one to observe such a thing; Julian Castro observed it during a primary debate, and received severe backlash for it. In this sense, the debate didn't really reveal anything new, it just confirmed something a lot of people had already expected: Joe Biden, as an 81-year-old, is currently undergoing entirely normal and predictable cognitive decline.
A lot of Democrats are fond of saying that they would vote for, e.g., a literal bucket of sludge before voting for Trump. This is an entirely reasonable and valid position -- I mean it. But what you have to understand is that many Americans do not think this way; for better or worse, "is cognitively capable of performing the duties of the Presidency" is a non-negotiable requirement for many people. Polls are consistently showing that Biden suffered a real loss post-debate. Over 70% (!) of Americans believe he does not possess the cognitive faculties to be President. This should be extraordinarily alarming for anyone who doesn't want Trump to be President again.
The good news is, it doesn't have to be this way. Several people have expressed that changing candidates to Kamala Harris would make the Democrats look weak. This is certainly possible. And Harris definitely has her flaws, as a politician and as a candidate. But -- and this is important -- far fewer than 70% of Americans think she is literally mentally incapable of being President. This alone makes her an infinitely better candidate than Biden, because unlike Biden, she can actually win the election.
Think of it this way: do you think Joe Biden, of all people, is going to be able to avoid another gaffe or poor debate moment until Election Day? Because the next time he does something that can even be interpreted as a sign of cognitive decline, expect the current news cycle but even worse.
This website once proudly referred to itself as the "reality-based community." But I don't see much reality these days. I see desperate emotional appeals in order to cover up what anybody who talks to a real person or can look at a poll could figure out: Joe Biden cannot win. The people who are really helping Trump are the ones insisting that Biden must remain in the race, because Biden cannot win. The only way to stop Trump is to add your voice to the many, many Democrats demanding he resign until it becomes so overwhelming even he can't ignore it. Maybe it won't work. Maybe Kamala Harris would also lose. But a chance at victory is better than certain defeat.
Biden is toast. But the country isn't. Not yet, at least.