Who do they want if Biden does step down?

[Metaphorical]

91. I expect this will happen

However, I can see the above scenario play out if there was an actual crisis with Biden.

Keep in mind, I doubt that the age aspect would be that critical if Trump was in fact in full compos mentos. Both Democrats and Republicans have had Presidents who were disabled for part of their term - Woodrow Wilson suffered a stroke in 1919 that left him paralyzed and hospitalized, with Edith Wilson effectively stepping in as de facto president. Franklin Delano Rooseveldt, had health problems all his life, and ultimately died of a stroke at the fairly young age of 63, Reagan had Alzheimers throughout much of his second term. Biden is, by all indications, remarkably fit for someone in his 80s, and I suspect that while he's probably on the same kind of regimen of heart medications that I am, he has a good chance of living to 100.

Let's also talk about Trump. The man has lost a lot of weight, likely through Ozembic, which likely has placed significant stress on his heart, he likely has one or more STDs, has high arterial build-up from a poor diet, has had a fairly extreme case of Covid, and is under a huge amount of stress right now. Take away the spray tan and hair coloring (and extensions), he looks like an old man in his 90s. Moreover, he has a family history of demantia and Alzheimers, and is presenting more and more signs that this is accelerating. The GOP is not going to replace Donald Trump - not now that he has his claws in the organization.

There's a lot of projection going on in the media right now, deliberately NOT looking at how ill Trump is by turning this into a debate about Biden's age, and one bad outing (it happens, especially if you are dealing with a demanding schedule) should not be sufficient reason to have him step aside. Frankly, I think the media's going to try to fan the flames as much as they can, but Biden's continued strong showing AFTER the debate is making that much harder to do convincingly.