There is no question that Biden is facing an uphill battle, just as much, if not more so, as Trump....I think you have it backwards. Just my opinion.
He made a very big mistake in dismissing RFK, the way that he did. If hehad been given secret service protection, and the opportunity to debate, I doubt that RFK would have run as an independent. Ralph Nader, by running on the Green Party ticket, in 2000, threw the election to Bush, causing Gore to lose by 96,000 votes. The same could happen if more Democrats are disgusted enough with Biden to vote for RFK - and Gore wasn't disliked nearly as much as Biden is.
RFK is far too left-leaning for the majority of Republicans.
Disgruntled Democrats who are sick of Biden, and would prefer a Progressive candidate, have the choice of voting for Cornel West.