Just the way I see it, and I'm neutral on the Dems, fully supporting Trump tough on China and border security right from day one. My motto is we need a strong, vibrant USA or the West is finished.
Thus, I believe Bernie supporters feel they aren't a member of the the Democrat party, though they were massaged to believe they were for months. Maybe they won't vote for Trump, but many of them won't vote for Biden either. They know he has huge following so instead of wanting to basically anger them they gave them some oxygen, expecting them to get in line. Once Bernie kept himself on the ballot they had to shut down any leverage they might have if he keeps picking up candidates (or, for some reason Biden doesn't go forward and Bernie can't say "I have ______ # of delegates).
This is the problem when so many embraced the alt-left with a "the enemy of my enemy is my friend" approach. CNN and particularly MSNBC were all too happy to embrace AOC and crazy extremist political correctness and policies that they knew a small number of Americans support, and others that offended everyone but the most indoctrinated.
Consider, when you go after Kavanaugh without accepting basic premises of Due Process and Presumption of Innocence (some even waving off these ideals), when you say "believe the woman, always!" regardless of the situation, simply because it fits your narrative and power ambitions at the time (be it the SCOTUS or otherwise) it ALWAYS backfires. Just one example of many of the new "Cancel Culture". If hurts them and all of society, though they often don't understand this.
This is why some of us try to warn you all about what's on the other side of socialist. The socialist way is absolutely NOT the American Way. First it's small steps to silence you, then it's big steps and demands, large government interference in the economy, then in your lives and that of your family. Ultimately you will be living a Kafkaesque existence if socialism wins and you will be judged or destroyed at your Overlords whim.
Bernie is a worse choice than Biden, however, don't tell his supporters that. They are deeply invested now, many of them fooled into believing they had a chance, MSM recruited them for their anti-Trump purposes when it served them. Now, they are tossed away in a manner that suggest "you REALLY thought we took you serious!? LOL!". Many of them will reject Biden out of spite. Others, will support Trump for his trade policies and anti-establishment representation.
The elimination of even their ability to vote will be the last straw and a boon to Trumps campaign. You can be sure he will get full value when the time comes.