If the Democratic party can't get Biden elected over Trump, what makes you think they could get someone else elected?

The fact that the "someone else" wouldn't be 81 and have had that debate performance.

Look, one of Biden's biggest negatives going in to the debate was voter concern about his age. A large proportion of voters feared he was too old to do the job of President. He then went out and spent 15-20 minutes acting too enfeebled to do the job of President.

At this point, any "replacement level" Democrat would be a better candidate than Biden - because they wouldn't be saddled with that. Harris, Whitmer, Newsome, Buttigieg - whomever. They'd have their flaws and their negatives, but nothing as crippling as Biden's problems.

The fact that all the other Democrats have the same "to do" list as Biden doesn't change that. This was a catastrophic negative for Biden personally. He went out in front of 50 million viewers and acted as though everything that voters feared and that the GOP has been whispering about him for the last year or two was true.

Now the Democratic party seems to be losing their collective minds because they nominated an old guy who occasionally acts old.

This wasn't an old guy acting old. This was an old guy acting as though he lacked basic mental acuity. There is a difference, which is why nearly every political observer is acting differently than when Biden just walks stiff or has a gaffe or uses an old-timey anecdote wrong.