"Swallowing camels and straining at fleas"...You're definitely entitled to your opinion, given that the "mainstream" of the Democratic party overwhelmingly chose Biden. I doubt the facts support your "center right" characterization, unless the mainstream of the Dems is also center right. If that's true then letting the social democracy minority tail of the party wag the whole dog makes little sense. I also dispute that the average policy position of the Democratic voter is at all to the left of Biden.

Right now Biden is trying to overcome the anti-democratic slant in the electoral college, a slant that can still hand the election to Trump even if Biden wins the popular vote by 10-15 million votes. At worst we have a choice between a dog shit sandwich and a P B and J. At best we have the opportunity to elect a life long and committed progressive, to at least preserve 5/4 split in the SCOTUS, maybe to flip that balance to the left, to return to the fight against global warming, for voter rights, against increasing wealth inequality, against color discrimination, LBGTQ discrimination, it's an easy choice.