I must say I find Sanders supporters delusion rather staggering. I get it, you like the candidate. But seriously:
1) The idea only Sanders can beat Trump has no basis. If he can't even beat Biden amongst a further left leaning electorate in the primary, how on earth do you expect him to beat Trump?
2) The idea Sanders increases turnout looks demonstrably false - in places he's doing well, turnout hasn't increased, where turnout has increased its gone massively to Biden. If anything the opposite is true, it is Biden who is turning out voters.
3) He's been robbed by the DNC. He's just not. On the one hand he wants to other himself and paint the rest as corrupt establishment, on the other his supporters are upset when those people he's specifically distanced himself from endorse one of that group? Regardless of which if the only way he could win is through 5 moderates running he clearly isn't a strong candidate.
4) The idea Biden is unelectable. Again zero basis. He does if anything slightly better than Sanders in head to heads with Trump, not that they're much use this far out. He's turning out voters better than Sanders. His coalition of voters is broader. More importantly he turns out people who actually vote. I don't think he's a great candidate either, but he's clearly stirring something up in the turnout.
Honestly, I get it, Sanders has some cool policies. But the constant conspiracies if he's not handed the elections are bizarre. The US has never been a left leaning country, the evidence is it still isn't.