They did it in ’08. They switched en masse to Barack Obama when it became clear who was going to have a majority of pledged delegates. In that sense, Bernie was absolutely correct. They’re irrelevant in the scheme of things. They won’t overturn a pledged delegate majority.
Again, you’re conflating a probable outcome with a definite outcome. Bernie’s the underdog, sure, but that’s why we hold the contest instead of just giving the prize to whoever’s the favourite.