Bernie's theory of change means that just winning the election isn't enough. He needs mass mobilization and civic engagement from millions of people during his entire time in office. So sure, they'll vote for him, but are they going to rally and march on Capitol Hill for him? Are they going to vote to primary Cory Booker and Kamala Harris for him? If you don't do any outreach to this wing of the left, they're not going to go the extra mile that Bernie needs them to go.
I'm concerned that a lot of Bernie supporters and the left are so addicted to being losers that they don't actually know how to govern. A lot of these people would rather sit on twitter and complain about the libs and cook up DNC conspiracy theories than actually create the coalitions needed to govern and enact progressive change. Social media has really fukked up our politics and culture.