All you have to do to defeat that is to fucking vote. Bernie lost the Presidential primaries in 2016 ans 2020 because more people voted for Clinton and Biden.
Yeah, there was talk about them rigging the 2016 primary for Clinton with the superdelegates if Bernie was ahead, but it never reached that point because Clinton secured the nomination before the convention.
I teach at a university and I had a ton of students praising Bernie who couldn't be assed to change their registration to a local address or drive home for a day to vote for him. And then they refused to vote in the general because they were protesting that the person they couldn't be bothered to vote for didn't win the nomination.