During his campaign he has paid for numerous polls to check the scenario. As far as I know, he won every one of those, which was not the case for Clinton.
I believe that it is actually highly plausible, as in my opinion the whole election was spinned to one question - "change or no change". It was damn hard for Clinton to prove she's for change, which would not be the case with Sanders.
Given that (I think) every poll up to November had Clinton handily winning the presidency, I think we can probably take those polls with a grain of salt when it comes to "proof".
Bernie did use those polls throughout his campaign to paint himself as better oponent to Trump, so I do not believe those were without merit. His main argument then was that he wins with Trump every time while Clinton does not. We can of course discuss merit of those polls, but as I have written earlier, it fits with my view of that election and the reasons Donald Trump became POTUS. You are free to disagree of course, this is my opinion, not a fact.
