I agree, @Chocoloca. I think Biden is in a fluky situation, partly based on the dismal way American politics function. Traditionally, the vice-president gets a leg up for the next time around, because people think that person already has some sense of the job. The problem is two-fold. People figured, and statistics agreed, that Biden was the only one who could beat Trump again, although, granted, those polls were a year or two ago. Second - and I stress that this is my personal idiosyncratic opinion, and no one agrees with me - Kamala Harris has been a bad choice for Vice President. Biden made a classic political deal, foolishly agreeing to choose a black woman as VP in exchange for black support. He fulfilled that deal, doubly so, with also appointing the first black woman to the Supreme Court. But I didn't much like Kamala before, and even less now. She was very competent in her previous position, but not here. I believe he needs a different VP, although, as I said, I haven't found a single person who agrees with me.
But, getting back to what you said, Democrats don't seem to have been brought forward - or maybe they're too busy doing their jobs so that they don't bother with the grand-standing that Trumpers do. There are a bunch of names floated as a replacement president - and almost any of them would be better than Kamala as VP, which is how I think it should go.
You make a good point that, logically, a president is not required to come up with solutions in seconds. Moreover, he doesn't have to do it singlehandedly, and one wonderful thing about Biden is that he has good advisors. (Trump, on the other hand, had a tendency to hire incompetent cronies, and the better people left or were fired.) But maybe Americans are trapped in a paranoia of something needing an instant one-man decision - news that Russia has launched missiles in the middle of the night - and they cling fanatically to that fantasy.