The legality is not very interesting imo. Monopoly law has been absolutely gutted by Reagan, and weakened by every single (yes, every single) administration since.

The question of whether having a monopoly position and not abusing it is something we as a society should do something about is, to me, equivalent to the following. If someone owns an automatic rifle, but isn't shooting or threatening anyone yet (i.e., using it to harm society), should we do something about it? What about a bazooka? More abstractly: should society aim to prevent harm when reasonably possible, or only punish it after the harm is already done?