Which I think plays 10x worse than being called a socialist.

Biden doesn't seem to have the energy (or the will) to push back in real time against Trump. Trump will bully him. And contrary to popular belief, Americans love a bully.

Sanders still runs around like a spring chicken and will finger wag Trump to death.

Also policy discussion wise, I think Sanders would has the advantage over Biden against Trump. Yeah, his proposals are "extreme". But they are generally popular.

How is Trump going to attack Sanders other than to just raise the vague specter of socialism? Is he going to say people shouldn't have healthcare? Is he going to say billionaires should be taxed less? Is he going to say people should be paid a lower wage?

With Biden, I can't even verbalize what he is striving for other than bringing us back to respectability.
He is on record vigorously supporting cutting social security and medicare and Trump will hit him hard on this (no matter how hypocritical). Trump will call him corrupt over his son's Ukraine stuff (no matter how hypocritical).

I don't care who actually becomes president, but I just see Biden as a much weaker candidate versus Trump.