These Sanders supporters already feel slighted by the DNC in the way Biden suddenly surged to take the nomination.
If they replace Biden with anyone but Sanders, this support is gone from the DNC for a generation.
That is why they must hold together and keep Biden together.
The Democratic Party depends on this....at least the Clinton/Obama cabal running the party needs this. They cannot turn the party over to the Sanders wing, which would purge all the Clinton/Obama people from their jobs.
Twice now.