The fact is that only a small minority of Sanders supporters failed to vote for Clinton, and most of those were probably in the states Clinton won easily anyway, so irrelevant. The other fact is that Clinton failed to generate enough support from independents to decisively win. If Biden loses it's because he's a weak bumbling candidate and the Dems will have no one to blame but themselves, which of course won't stop them from blaming Sanders and his supporters anyway. Whatever it takes for them to avoid facing reality instead of adopting policies and candidates that would actually be popular.
An unusually large poll done in early 2017 (several thousand people polled) found that the percentage of Bernie supporters who didn't vote for Clinton was a bit larger than usual from this sort of situation, but 75% did vote for Clinton. Of the 25% who didn't, about half voted third party or not at all and about half voted for Trump.
Of those who voted for Trump, at least some had intended to vote for Trump all along and were strategically voting in the Democratic primary to try and weaken Hillary. And there were some Bernie voters who were so angry at the system they just wanted to make some kind of dramatic change and didn't really care what direction. They were more intent on throwing a wrench in the works than actually accomplishing any change.
But still 3/4 of Bernie supporters did hold their nose and voted for Clinton. It will likely be larger this time because in 2016 a lot of people figured Clinton had it in the bag and wanted to make a protest vote for someone else and the Democratic candidate is far more likeable. Very few people actually like Hillary Clinton, while even quite a few Republicans like Biden personally.
As far as how much Bernie campaigned for Clinton in 2016 vs how much Clinton campaigned for Obama in 2008... Obama didn't really need as much flank support as Clinton did. Obama is a charismatic campaigner who sold himself. Hillary is a boring policy wonk who couldn't sell water to someone dying of thirst.
In the Democratic primaries, the primary voters want to hear policy positions because there are quite a few Democratic primary voters who are policy wonks themselves. Especially those talking about the primaries in the media. But come the general election, the bulk of the voting population don't care about policy. They are moved far more by emotion than by cerebral arguments. The only time policy is an issue in a general election is when a candidate has long standing positions that their opponent can use against them and it becomes an emotional issue to the voters. The outrage that candidate A stands for X.
Hillary lost because she was in a race with someone as equally unlikable as her, but who was more interesting. Kerry lost because he was another boring policy wonk and what put the nail in the coffin was the Swift Boat ads and Kerry's failure to combat it early.
The 2 most important factors in a candidate in modern politics are likability and are they interesting. And likeability is most important. Trump is still more interesting, but he's turning out to be as interesting as a series of operations to remove shrapnel from stepping on a land mine. Biden is definitely more likable.
I think Bernie's attitude about Biden is the approach the entire extreme left should take, Biden is to the right of Bernie's positions, but he's also open to suggestions. Any Republican is going to automatically ignore anything coming from the left, but a left of center Democrat like Biden is much more likely to consider ideas from the far left as long as they appear to be workable in the real world. Better half a loaf than none at all.
