They could 25th Amendment Biden and remove him from office. There are several problems with that. First, it would be an admission that they were covering up Biden's decline in mental ability. Second, according to DNC rules, it's almost impossible to remove a candidate without their permission. If Biden doesn't formally drop out of the race, he would still be the nominee even if the 25th Amendment removes him from office.

There are problems with not removing Biden from the ballot. First, it's going to be nearly-impossible for him to fundraise now. Nobody wants to give money to a candidate that is going to be replaced, or is sure to lose if not replaced. Second, he's likely to lose. If there's a second debate and Biden stumbles through it again, that would be the end of the election. Every public gaffe Biden makes from now on is going to be max scrutinized, instead of merely being ridiculed on a handful of right-wing websites nobody else reads.

2020 primaries

Actually, Sanders had a big delegate lead (superdelegates don't count). Then the primaries had to be cancelled due to COVID. Biden somehow won a single primary and then everyone else dropped out of the race (including Sanders) and declared Biden the winner.

RFK

If he is on the ballot as a 3rd party candidate, he takes more votes from Biden than Trump, giving Trump an easy win. It's similar to how Perot cost Bush the election in 1992 even though Perot had no realistic chance of winning himself, and how Nadler cost Gore the election in 2000. (If Gore got 80% of Nadler's votes, he wins without a recount. Presumably, every Nadler voter preferred Gore to Bush.)