There's a strong 'Never Sanders' crowd that I'd like to test your argument with PIA.
The most vociferous of that lot seem to be the ones who oppose him because he's not a Democrat and hence should not be the "Democratic" candidate. The same ones have a deepseated issue with the word "socialism". But, as a group, they are conditioned to vote in every election. They are the first to shout "Vote Blue No Matter Who!" when they want you to vote for their candidate and they will be the last to sit home when there is a chance to do their civic duty. They'll vote Sanders if The Fates should weave that thread.
Loyalist Democrats one and all. But they'll be voting en masse in the primaries. They'll be deciding who gets the nom-noms.
Lefties wanna take over the country? They need to overwhelm this crowd in the primaries.
"Be yourself; everyone else is already taken."— Oscar Wilde