Please do away with this old, debunked trope.
"Bernie supporters" ... there are two kinds of these.
1) Democrat Bernie supporters who voted to a huge degree for Hillary in the last election, and even if those, who voted for Jill Stein instead had voted for Hillary, it still would not have been enough. They did NOT lose Hillary the election. They also will vote for "any blue" this election.
2) People who care nothing for the Democratic party, but who were willing to go to the polls for Bernie, because they hoped for true change. So they either stayed at home or voted for Dump, who also promised "change" of some sort.
Why did you expect these to come and vote for Hillary? What did she have to offer?
And, more importantly, what has Biden to offer to those (other than the real, tangible advantage of getting rid of Dump, which we did not have in 2016 and which will hopefully motivate many more to go out and vote this year)
That, in a nutshell, is what people who are genuinely anxious about Biden being the front-runner now are trying to tell you....