Sanders is misreading the situation. Many of his supporters are frustrated with him for treating Biden with kid gloves during the campaign, for endorsing Biden and calling Biden his friend, and for his votes on the bad coronavirus bailouts. Many of his supporters have already moved on from Sanders and are planning their next moves. Support for Sanders was always conditional on him pushing ideas his supporters cared about. When he started stumping for Clinton and then Tom Perez, his support dropped. The same thing is happening now. If he were to run for the Democraticy Party nominatino for the third time, he won't have nearly the popular support he had the first 2 for that reason.
I'm sure this does describe some part of Sanders' supporters. For example, YouTuber Kyle Kulinski takes approximately this position. My question is how many of former Sanders supporters will take this hard line?
I think that over half of them are regular Democratic party voters who wanted progressive policies, but will fairly happily vote for any Democratic candidate. There's also another pretty large bunch who feel sad and somewhat betrayed, but will eventually follow Chomsky's advice and vote to defeat Trump.
In my opinion, the hard rejectionists could be anywhere from 5% to 20% of Sanders supporters, and the exact proportion will be crucial in determining the result next November.