Monopoly is a red herring. We should be going after trusts of all sorts, even if they don't rise to the level of monopoly power. A cartel is no less damaging just because the individual players don't hold monopoly power. What matters is that you leverage your large market position into political power used to enact anti-consumer policy. Competition is only important in-so-far as it is capable of protecting consumers, so even if there's a fig leaf of competition, that's not the point if it isn't effective at protecting consumers.