Well, there's another option - they are allowed to arbiter, but aren't allowed to control the rules on the arbitration, at least not directly - instead, an overriding entity would do that. The entity would be, most likely, the government, but as a window dressing, an "independent body" whose members are selected by the politicians can be established to give it a veneer of being removed from everyday politics (protip: it never is). While not a lot of people would admit to preferring this model now, I suspect, a lot of them are pushing in the direction that can't lead anywhere but there. Of course, once such body is established, its natural evolution would be first to control all large public speech platforms, and then to attempt to control all speech in general.