I disagree with your characterization of "the official narrative" I feel like "Russian Interference" is another thing that mixed in there for example. But even putting that aside... That's only in your subjective view, which is predisposed/biased towards seeing it this way, because of your deep preference for Sanders.
If I was interpreting the 2016 election through a lens of relentless pro-Sanders bias, I would surely prefer Clinton's defeat to represent a damning rejection of her politics by the American voters, and not a fluke generated by your dumb eighteenth century electoral system.
I don't imagine that Sanders would have out-performed Clinton by sweeping margins. It's plausible that, in terms of the popular vote, he may actually have done
worse. But it has widely been observed that what pushed Trump over the finish line was the support of working class white voters in the Rust Belt who were not a natural Republican constituency. Even those who point to Russian electoral interference are largely arguing that this interference allowed Trump to win these voters. Sanders would appear to have been better-positioned than Clinton to sway these voters to the Democratic Party. If this is untrue, it is because there are other voters of comparable strategic importance that Sanders would turn away. Do you have a proposal for who that might have been?
Yes. He lost and has all but lost again to Biden. What better proof are you looking for? I can believe with all my heart that my favourite ball club is the greatest team that god ever gave man, but my belief will not deliver them to victory. If the other team is better, they will lose. My response shouldn't be to claim that their loss proves that the game is flawed, rather than considering that my team just wasn't as good as I thought.
Biden has been successfully because he has successfully mobilised Democratic Party loyalists. His core constituency was people who were almost certainly going to vote, and almost certainly going to vote Democratic. The chances of Biden pimary voters switching to the Republican ticket in the general are limited; the chances of Biden primary voters switching to Trump in the general are, let's be technically precise and say "non-zero".
This has been possible because the Sanders campaign has failed to mobilise as many marginal or disengaged voters as they hope, or I daresay expected. This is undeniable, and it casts doubt on his campaign's potential to achieve this sort of mobilisation in the general, which was always part of his pitch. It is not good for Sanders. But the way in which Biden has achieved his lead in the Democratic primary, by marshalling the highest number of inevitable Democratic voters, does not point in any clear way to his achieving a lead in the general. His campaign has proven even less convincingly than Sanders that it can win the support of people who otherwise would have voted for Trump, or would have abstained altogether, and therefore that it can avoid repeating the failures of the Clinton campaign.
If Sanders' campaign is doomed from the start, then the primary is nothing so much as two men arguing about exactly how they should lose the election, and you are simply expressing your preference for the candidate who proposes to lose the same way they always do. To believe otherwise, you must accept that Sanders has the potential to win in the general, even if you maintain that he has is less well-positioned to do so than Biden, or you must contend that Biden is better-positioned than Sanders to mobilise marginal or disengaged voters, despite having no tangible record of doing so.
The only way you can maintain simultaneously that Sanders is a no-hoper but that Biden is poised to take a majority in the electoral college is to assume that large numbers of Republicans will defect in the general. But that is to assume that people who were willing to swallow their distaste for Trump in 2016, who were prepared to support him long after it had become clear what a comical monstrosity he was, should change their minds at this point, after his administration has shown itself to be far more moderate and more cooperative with congressional Republicans that many assumed it would be. Is that really credible?
If everyone is disenfranchised, then why the heck is Sanders running? Why not go straight to the armed revolution?
The movement around Sanders is quite explicitly an effort to enfranchise working class Americans. This is like asking "If you're already hungry, why bother eating?"