"Should Biden step aside?" is a bad discourse because he's almost certainly not going to--literally everything about him tells us that he is not going to--but "he's not going to" is not actually an argument that we wouldn't be better off if he did
"Replacing him with Harris would not ensure certain victory over Trump" is a true statement, but "whereas Biden's victory over Trump, on his current trajectory, is much more certain" is something often strongly implied without really substantiating, and I'm pretty skeptical of it.
One reason for the post-debate despair is that a lot of people's hope that Biden would win already had "his numbers will rise after the debate" baked into their calculations. Without it, we're left with a dude who seems to be doing significantly worse than he did in 2020.
I fear/suspect that Biden will lose, and it will be very bad when he does, and while everyone will say "he lost because he was very old," the reason will have been, basically, Gaza. On some level, people were hoping the debate would have fixed that, and it didn't.
It might not be quite as simple as "the number of Democrats who voted 'uncommitted' in the Michigan primary represents at least the margin of his 2020 victory and if he loses Michigan he's probably done" but it also might be
And Netanyahu hasn't even told Congress yet that Biden stabbed him in the back, nor has Biden dragged us into Israel's upcoming regional war with Lebanon, lots of fun stuff ahead
