the real problem is that the democratic party rigged the system against a candidate who could have beat trump, and ended up with the only candidate who could have lost to him.
I think he would have done a good job at picking up votes from the "disaffected" crowd that voted Trump over Clinton as a protest, but there's certainly the possibility that he would be lose votes for being too far left.
At this point, I certainly still wish Bernie won, seeing as Trump is president regardless, but I don't think there's necessarily a solid counterfactual that he would've beaten Trump. Either way, I think some of the blame can certainly be placed on Democrats pushing more of the same when people were clearly calling for a change.
[1] https://fivethirtyeight.com/features/trump-is-just-a-normal-...