This and also, you have to assume the majority of Bernie supporters will vote for the nominee just like they did last time. Even if you have heads that stay home in 2016 proportions, Biden has shown he can bring in newish voters of his own in suburban areas. That's pretty much how he's winning all these states by wide margins.
And you know this makes fun of everytime but actual normal republicans who hate Trump just as much as you and me are willing to vote for Biden so I hope that in worse case, this offsets the Bernie people that stays away.