I mean, its unlikely that someone who voted for Trump last time will be voting for Biden this time. I think the Democratic party is missing the boat on this one. They're so busy pushing someone "presidential" that they are forgetting who the actual target audience is, Trump voters.
Don't you mean the Democratic primary voters are missing the boat? Bernie's dropping out because he simply didn't get enough primary votes.
Also, for all of Biden's problems, it's quite possible he'd do better than Bernie in the general election against Trump. Many undecided voters, and pretty much all past Trump supporters, strongly associate Bernie with communism, which is basically one level under "terrorism" in their mind.
I say all of this as a Bernie supporter. Democrats were going to be very likely to lose the election whether it was Bernie or Biden. Yang, Gabbard, and Klobuchar would possibly have a higher chance in the general election, but that's also unclear.
Why do you say that? Voter turnout during primaries was higher than 2016, and Biden was doing well. Even though it may seem like Bernie is more popular on the internet, Biden is more popular among people that actually vote (older people).