I think I've lost any remaining ability to be stunned by the ineptness of the Democratic Party, and it leads me to entertaining conspiratorial thoughts such as, perhaps the Dems realize that the shit is going to hit the fan and they don't want to be the ones in charge when it happens, so they foisted Old Man Biden on us in hopes that he'll fall against Trump.
Maybe not, but we can definitely be sure based on all the manuevering that was done back in late February/early March to give Biden the Super Tuesday win that they obviously would rather have Trump than Bernie Sanders and that's a pretty sad statement and makes me not really pay all their "Trump is an existential threat to all of us" talk too much attention. Not that I don't think a second term of Trump MAY be such a threat, but just if they really believed that, why freakin' Biden?
And yes, all they have to do is just pick one of those issues you mention---Medicare for All, student loan forgiveness, even legalizing weed---and that'd likely be enough to push them over the top, but they can't even do that.
A sad, sad state of affairs, indeed.