It's not an argument
I'm making; it's an argument lawyers for the DNC have effectively made in a court of law defending DNC against a class-action lawsuit by voters.
https://observer.com/2016/10/dnc-lawyers-argue-no-liability-..."Courts have uniformly rejected attempts to litigate on the basis of purported political promises, including ‘statements of principle and intent in the political realm’..."
The DNC can say it's unbiased, but the rules-as-written allow broad, sweeping bias in how the Democratic party conducts its affairs. It's a party. They're biased by default. The whole point of parties is, basically, as you put it, "The peasants don't know what is good for them." There's a reason so many politicians throughout history have been anti-party. Unfortunately, they stick around and grow in power because they tend to work; focused political alignment beats unfocused noise consistently.
What happened with the Sanders campaign is a whole bunch of voters crashed headlong into a system that works other than the way they assume it does.