I think it's not even about the election at this point, the Dems know they've embarrassed themselves for 4 years in regards to Trump and know the L is coming.
The difference is, Biden gets you one of those standard Al Gore, Mitt Romney style losses. The problem with Bernie is that once you jump off online echo chambers, socialism just isnt that popular. Not to mention all of the things Bernie has said in the past, like praising the USSR, the Sandasistas, and Venezuela publicly before they all crashed. The project veritas video of the Bernie Staffer saying he knows violence is necessary, saying we need gulags for reeducation, saying cities will burn if he doesn't get the nomination. "Bernie Bros" actually being the toxic, violent, reactionaries the media tried to tell you that Trump supporters were gonna be. There is a lot of ammo Republicans are sitting on and won't touch until Bernie gets the nomination.
If he wins the nomination, The Democrat party will become the party of Socialism and communism and not only will Bernie take the beatdown in the general election, Dems will lose ground in the house and Senate and possibly hand Trump a super majority.
TL/DR: Biden is the punt on 4th and 25