Here's my counterargument:
http://www.gallup.com/poll/183713/socialist-presidential-candidates-least-appealing.aspx
Now, maybe over the following year and change, Bernie could've completely turned that around! But in a campaign cycle where a significant portion of the electorate is deathly afraid of the
idea of Muslims, the fact that Americans would be inclined to vote for a Muslim president than a socialist one seems like Bernie would have faced a giant uphill battle.
And as previously pointed out, Bernie being the nominee would've meant his campaign would be run by Jeff Weaver, who is totally incompetent. He would've never put together the same coalition-building, data-driven GOTV effort that Mook is overseeing -- and clearly didn't, as evidenced by Hillary winning in a landslide over Bernie.