The official narrative is that Clinton only lost because of disaffected blue collar workers in the Midwest, who were drawn to Trump's populism and who disliked Clinton personally. On both counts, Sanders would have appear to represent a more competitive candidate for those voters specifically.
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.
This would not hold true if we believe that Sanders would have lost a strategically significant portion of Clinton voters to Trump, or to abstention. Is there reason to believe this may have been the case?
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.
Tying this particular gripe to the discussion about political power smacks hard of sour grapes. In other words, when Sanders is viable, and his winning seems possible, we're attending rallies and advocating that people vote for him
within the existing system and talking about how he's such a great candidate because all the young people and/or all the hardworking, blue-collar, Midwest etc., folks would exercise their immense power
within the current system to carry him to victory... but then when he loses, the argument becomes that the people never had any such power in the first place and were essentially completely disenfranchised... the whole process was impossible from the beginning and you never wanted any part of it anyway.
Sanders supporter: "Exercise your political power and Vote for Bernie!"
Voter: "No"
Sanders supporter: "You obviously never had any political power, you're disenfranchised"
If everyone is disenfranchised, then why the heck is Sanders running? Why not go straight to the armed revolution?