You are right if we roll back the clock to 2016. Once old Bern threw in with Clinton many people stopped believing he was a fighter. If he won't stand up for himself how can he be expected to stand up for those who support him?
Sanders has no choice but to do what ever is necessary to run as Democrap. The system is rigged so a 3rd party has zero chance. It is the 12th amendment. No one reaches 270 electoral votes in a 3 way race, then "congress picks" the president from top 3 candidates. Congressmen from each state vote amongst themselves for whoever they want. Each state then gets one vote. If that state does not have a majority of congressmen from that 3rd party, there is a snowballs chance in hell of them getting elected. So a majority, in a majority of states is needed by that 3rd party. How can that ever happen.
There is a contract that says they must support whoever the nominee is. He violates that, then can not hijack the Democrap party.