I think that both what you and the other commenter have said is probably pretty accurate, but I also wanted to note that the office of the president is immensely powerful on its own.
Executive actions can have immediate changes without support from others. And having someone like Bernie in a position to set the national tone by being ‘the face’ of our politics for 4 years could certainly shift the Overton window. Bernie speaks his mind, and as the leader of the U.S., folks listen, even if he can’t pass policy. So if he spends 6 months leading up to an election talking about preference voting, ethics reform, or campaign finance - candidates and primary challengers are going to discuss those positions rather than leave them off their platform.
Those issues could shift the baseline of political power in the U.S. - if we had an election cycle or two where the president was driving national dialogue about it, voters would also choose candidates who made those things a priority - empowering - perhaps not Bernie, but maybe the next round of candidates, and possibly leading to permanent changes to our democracy.

Which is, well, very hopeful. But it illustrates what he could have done. Could have changed trajectories, rather than circling the same old drain that leads to a two party system where both parties are on the take from someone.