Putting aside the fantasy that anyone's lives would actually be noticeably different under any one of these Democratic candidates than any other...

The more egregious fantasy that you're putting forward is that if I openly give my heart and soul to the candidate who claims to plan on giving me what I want, then that "ever so slight" gain in their electability matters even when the other candidate has built-in advantages with the broader electorate. We dorks have another term for "ever so slight" in cases like this. That term is "negligible."

Imagine if there was an objective number to represent the "electability" of a candidate. (Yes, I know there isn't any such thing, but this is a premise that you assumed when you talked about making someone "ever so slightly" more electable.) Comparing such electability numbers for two candidates, if A<B, and epsilon<<|B-A|, then (A+epsilon)<B. Making one candidate "ever so slightly" more electable than they used to be does NOT change the fact that they're less electable than the other guy. (In fact, if our goal was to use our personal charisma to make some candidate more electable than they used to be, it would do more good to throw that weight behind the candidate who is ALREADY more electable. If we could actually make a candidate more electable, then we should all become Biden Bros.)

Note: Bernie himself sees this. This is why at every campaign appearance he harps on the fact that we need a massive grass-roots political revolution to bring about the kind of change that he envisions. On that, I wholeheartedly agree. If anyone was ACTUALLY gonna do half the things Bernie promises, we'd need to find some way to change those built-in electoral advantages for a candidate like Biden into advantages for a candidate like Sanders.

But here's the thing: that revolution has not come. Even for Bernie himself. He's had five years to push the grassroots to make his promises more popular and more believable. And now, just as before, he commands the grudging support of a little less than half of a little less than half of the overall electorate.

If your candidacy is contingent on a revolution, as Bernie says every day that his is, and you CAN'T bring that revolution, then you have nothing.