And here you touch on the real problem. By banning extreme people from the commons you send them all to cluster together somewhere else, where their views are met with support instead of derision, thus amplifying and intensifying the extremity of their views.

Steve Bannon on Facebook gets called out and shouted down. Steve Bannon on Parlor gets praised and encouraged. People on Parlor get positive feedback and begin to think their views are mainstream and reasonable. People on Facebook get negative feedback and at least have a chance to think "Hmm, maybe if everyone says I'm wrong about this, I should rethink it".

Yes, I know many people won't rethink it, but facing some opposition at least provides the possibility of that happening. Forcing everyone into a filter bubble of only their own views provides no possibility of that happening.

Facebook is already a filter bubble though. It is Facebook itself that can impose opposition.