His attempt at overthrowing the party revealed the contradictions of the Dem Party. He was necessary to show the futilism of electoralism in a dual-party system dominated by capital.

He also revealed that anyone will be corrupted by the institutions they claim to want to reform. He couldn't close the circle on his 'millionaires and billionaires' rhetoric. He should have pointed at Biden, Hillary, Obama, ect and said 'They're the problem. The party donors are the problem.' His friendships in the Senate and Dem Party made it impossible for him to fully crush the party. Which is really what he wanted. He wanted to take the party completely off the wealthy donor pipeline. The response from the donors is totally clear in retrospect, Sanders should have guessed that in the face of existential threat these people would circle the wagons and do anything to stop him.

He never made clear why the establishment and media despised him so much. Trump was much better at this. At a debate the RNC donors booed him, and he turned to the camera and said 'They boo me because they can't buy me. I'm not like the others.' But Sanders doesn't have the same gut-level instinct/animal cunning Trump possesses.