Antitrust doesn't violate the first amendment, so clearly limits on corporate scale aren't unconstitutional, so the legal defense is insufficient and the moral question stands.
> I'm reasonably skeptical that they do, given that the 1st Amendment hasn't stopped us from enforcing antitrust and monopoly legislation in the past. Yeah yeah, Citizens United and all that, but we regulate companies all the time.
> But I'd still want an actual lawyer to weigh in on that, I wouldn't feel confident saying that there aren't limits on how far we can go in that direction.
It doesn't necessarily hold that because one thing is legal, everything is legal. For example, we have 1st Amendment restrictions on threats and libel, but in the US hate speech is still protected speech. 1st Amendment exceptions are generally pretty narrow and specific in the US.
In the same way, clearly some corporate regulation is OK. It does not follow that there's literally no limit on what the government can dictate about how a company can operate. I would prefer to get input from a lawyer before asserting that so confidently.
