> If they were separate companies, it would require actual collusion a
I don't play bridge, never learned. A game consists of two teams of two players each. My vague understanding is that you might win if a partner gets ahold of a card that you discard, but that you can't know what card that is without saying so (cheating).
Good players can tell what cards their partners might need, and discard anyway. This isn't cheating, it's just good play.
In a court of law, the former is definitely some violation of antitrust law. But the latter is just good business. And even if it weren't, no overt acts have been committed that could lead to successful prosecutorial outcomes.
Philosophically, it may still be collusion. Good luck doing anything about it.
> It's also be less likely to occur since they'd need some motivation to do so.
Game theory supplies that just fine. If I have a monopoly in A, and you a monopoly in B, then I might just protect your monopoly in B without you requesting that, without you giving me instructions.
And from that point on, if you see a place where you can hurt or help me, you might choose to help without me requesting it or asking. Because if my monopoly on A is hurt... I can no longer afford to help you without you asking. Supporting me is a no-brainer. At least if the other party isn't a complete imbecile, they can see it. While there's no accounting for stupidity (second most powerful force after compound interest), we can already assume that the leadership of these companies is non-stupid just because of the success they've already achieved.
> Why would the shower curtain company even attempt to collude with the bowling ball waxing business?
When they break these companies up, they don't fire 100% of management and rehire. Most of them know each other. Most are friendly with each other, if not friends. Sure, employee attrition will eventually break those bonds ten years down the road. But they already know how each other thinks.
Silent collusion, without so much as a wink or a nod, is definitely within the realm of possibility. The previously existing social connections are present. Their goals were largely aligned before, and nothing has occurred to de-align them. Mutual success may be easier than individual success. A sort of "corporate altruism" can emerge.
Evolution does this shit all the time with symbiotes. It's not like their colluding to co-evolve with each other. You won't find conspiracy evidence in the fossil record that they just started helping each other, it only proved to be a better strategy than individuality, and so they went with it.