Those two are too far left I think. I am working on the assumption that Biden wins Pennsylvania and one or more of Michigan, Wisconsin, and Ohio. While Trump won, the Electoral College belies the kind of a squeaker it was and also how historically unpopular both candidates were.
Trump won by appealing to deplorables who otherwise don't turn up. A socialist like Warren - with a sack of freebies - would equally attract the fringe - students, progressives, independents, etc - who are repelled by mainstream politics. Hillary was a boring business-as-usual veteran that ensured low liberal turnout. There are lots of climate-loving, Green Party backers and tree-hugging hippies out there. Look at Germany or the UK. Assume GOP is the Tories. Democrats are Labor. Brexit Party and Lib Dems are the outliers attracted to radicals. In a duopoly these folks are disillusioned and need crazy ideas to excite them.
Yes, he appealed to those types to turn out. But even then, he only barely managed to win, aided by Hillary's unpopularity. That approach can only work for Republicans because they can be tribal. While Democrats have a bigger pool to work with, it's a less tribal and more diverse mix. So they cannot push one extreme issue like Republicans.
The liberal "tribes" are inequality, student debt, medical bills, climate change, etc. Sanders is a brazen socialist preaching 60% taxes. Warren wants a 30 trillion dollar free medical scheme and entitlements bonanza. Biden is Hillary's twin who will nail the battle and lose the war.
Bernie is an old dog who should step down for Warren. After he drops off his crowd will flock to Warren. Warren can beat Biden and take on Trump without suppressing turnout like Hillary. I hope the DNC learned the lesson not to bungle the primaries by favoring the mainstream candidate. Rigging in Biden is the surest path to defeat in the general elections.
Warren at the top of the ticket will not attract enough independents though. It's easier to get the various tribes to settle for the lesser evil and just show up for Biden.
Why couldn't the independents save the moderate Hillary? As a centrist Biden is worse. Hillary was leading the primaries by a mile but with her "blue wall" still lost to Trump. Those all-important swing tribes - deplorables, misfits, hippies, students - will fall for Trump or Warren or Sanders. Not Biden. A radical exciting the primaries is a good counterpunch to Trump. Bernie trailed Hillary throughout while Warren is leading. Biden is boring safe pair of hands.