There's no reasoning with these people. They're children. We basically let Bernie write the platform in 2016, an unprecedented thing, especially because he lost so thoroughly. They still threw a fit and almost ruined the DNC with their booing and protests. Now they feel like they deserve to lead the party because... uh... they came in 2nd in the 2016 primaries? It's not even a policy thing, Perez is as progressive as Ellison, they just hate anyone who dares to oppose their heroes.
But if they're children that can't be reasoned with, shouldn't we give them what they want? We're just going to lose again if they're not on board, and it won't be much consolation to take the moral high ground afterward.