Biden has one fatal flaw, though: He tends to screw up on the way to the finish line, that's how he lost his primaries before. Obama choose him as VP to show that he has a thick skin after the comments he made over him (his comment was: "I mean, you got the first mainstream African-American who is articulate and bright and clean and a nice-looking guy, ... I mean, that's a storybook, man."), and that could bite him hard if that gets digged out by right-wingers. And Trump is somebody who would exploit the hell out of his flaw, so not sure if he really would win against Trump in the end.
Bernie Sanders seems like a more probable winning candidate on the long run to me right now. Both him and Biden do win every national poll against Trump to date (which Warren, Harris or Booker can't claim), so he's certainly not just appealing the base. And he's known for staying steadfast to his convictions, so he won't flip-flop on issues, either, so that will be hard for Trump to play against him.
I do agree that Biden has potentially severe vulnerabilities. And the polls are really early now, seeing as the primaries don't even officially begin for another ten months. Things could change a lot between then and now in the polling. Back in 2007, Giuliani was the GOP front-runner in the polling for the 2008 primaries, and hypothetical polls from late winter/early spring showed him even with or slightly ahead of Obama, who himself was polling in a distant second behind Hillary Clinton. So yeah, things can change drastically. But at the moment, it appears that Biden, perhaps because of his status as Obama's VP, is doing best in the polls.
I do think that Sanders is much more solid candidate, one that I agree with on the issues much more than I do with just about any Democratic presidential candidate. As you said, the man has stuck by his principles for decades, and I think he honestly wants to do the right thing by Americans, and his big policy plans are popular (or have the capacity to be popular) with a broad swath of Americans. However, my main concerns are A) his age, and B) the fact that he is a self-described "socialist."
Now, Trump ain't exactly a spring chicken, but he was "only" 70 when he was inaugurated. Sanders would be 79. While Sanders could live to be 108 for all we know, his age could be turned against him. While there are clear rules for presidential succession, Republicans could still stoke fears that if Sanders was elected he could croak mere months into his first term because of his age.
And the "socialist" thing gives Trump and the Republicans the opportunity to go full-tilt "Tail-gunner Joe McCarthy" on him and portray him as a dangerous communist/Marxist/Stalinist who wants to destroy America. Fears of Soviet-style communism still resonate with older voters to this day, and older voters are the ones who have the highest turnout rates. I wish Sanders wouldn't call himself a socialist. He isn't, because he is not pushing for public ownership of the means of production. He's not arguing that Walmart, Amazon, AT&T, Comcast, Apple, Berkshire Hathaway, Exxon Mobil, Ford Motors, etc., etc., ought to be nationalized or forcibly converted into some kind of worker-owned co-op. What he does advocate for is not the end of private for-profit business, but rather a strong safety net, policies to reduce income inequality, single-payer health insurance, taxpayer-subsidized college tuition, etc. Those things do not a Stalinist make, regardless of the paranoid ravings of right-wing pundits and representatives.
But Republicans have spent the last 40 years making a cottage industry out of red-baiting. It's a pastime for them, and right-wing pundits never tire of it. Everything they hate gets called "socialist." I especially find it funny when even major multi-billion dollar corporations get slapped with cliche Red Scare slanders. CNN has often been referred to as the "Communist News Network," as though we're expected to believe that CNN's parent company Time Warner (and Ted Turner's media conglomerate before it) is actually some sort of Leninist front. The red-baiting from the right long ago reached the point of self-parody. But it works. It keeps the base fearful of the Democrats, and may even convince a lot of independents that the Democrats are plotting to turn the U.S. into a communist autocracy. "Socialism" is still a very scary thing to most Americans, and Sanders' history of appropriating the term to describe himself (even though he's better described as social democrat) isn't going to do him any favors. I really wish he'd stop calling himself a socialist.
If Biden's gaffes and other potential closet-skeletons and Sanders' vulnerability to being cast as a dangerous near-octogenarian Marxist become serious issues as we approach the primaries, I can only hope some of the less prominent candidates with less potential baggage can become serious challengers.