Biden already has enormous amounts of delegates pledged to him.
But what happens if a delegate breaks his or her pledge? What do they do if the mucky mucks back home say everything is lost if Biden runs? How loyal are Biden’s delegates to him vs to the democrat party? If the decision is made to dump Biden, changing convention rules will be child’s play for them.
But what happens if a delegate breaks his or her pledge? Then they will have disenfranchised the voters based on their personal whim.
That wouldn't be a very good "democratic" message.