The Democrats knew this was going to happen. There's no way they couldn't. And I don't mean Democrat supporters, many of whom were vehement that Biden was fine like so many anecdotes in this thread recount. I mean the Democrat leadership, who manage his campaign and more than likely manage his presidency. Unlike the public, they have access to him. They have his medical records, the reports of his doctors and caregivers, everything. There's no way they didn't know this would happen if he debated.
They might start seeding support for a different candidate into their supporter's discourse after this, but they will have been planning for this outcome long ago. And when a left-leaning (left from a US Overton window) news platform hosts a debate that shows him up that badly and then publishes commentary like this, you have to wonder if that caused friction with the DNC or if they assented to it.
As the party starts singling out a replacement, the question I hope people start asking is why they didn't replace Biden earlier? Did they need to wait until the urgency of imminent elections made their new candidate more palatable? And if they don't replace Biden, why are they letting Trump win?