Well, I hear that a lot, mostly from younger Sanders supporters.
I think they seem to feel that their vote alone would've been enough to carry Sanders to victory, without regard to how older, more centrist and conservative Democrats would've voted.
There were a lot of things Sanders was proposing that we older, middle of the road type Dems either didn't find credible or downright disagreed with.
Then there was the plethora of negative ammo the Repugnants had at their disposal to throw at him.
He's a socialist, the anti-women articles he wrote, etc, etc, etc.
So the question then becomes one of how many older, centrist Dems would've stayed home or voted for Stein or even some other candidate?
I'm not convinced that the enthusiasm of the youthful Sanders crowd would've guaranteed a win for him.