Well the first problem with that is that the Democrats let him go through the primary process so it's kind of shitty to do that and then say we arbitarily disqualifed you at the end. The second point is that it would probably be the end of the Democratic Party. It's a move that is a guaranteed loss for them because Bernie would significantly split the votes with the Dems. Also I don't know if the Democrats really want a generation of young voters who run the gamat from male, female, white, black, hispanic, lgbt, straight etc to think you cancelled out their voice.
You can do it but you are basically saying "we'd rather Trump be President and we are okay crippling ourselves in the future". Also it would wreak havoc down ballot because the split would have a lot of pissed off Bernie voters abstaining from supporting Democrats, who if he was the Democratic nominee they'd likely vote down ballot for.
It would be a complete **** show and a way to basically say they don't care about a wing of the party they pretend to include.