> Isn't that approaching a monopoly / duopoly where regulation starts to make sense?

Or it might be a cartel (few players are coordinating for maximum revenue, are not competing, prices are fixed), but we usually won't let that stand either.

It's good to remember that Google and Apple (and others) have been found to collude before when they made secret no-poaching deals. They settled for $415m.

I don't know how that would work though, without breaking up the companies.