To say they are in price collusion implies a conspiratorial act against consumers.
And also, it's not a duopoly, since there's more players, Nintendo in particular.
But moreover...
The Sherman Anti-Trust Act can't be enforced without proof of artificial restriction. Humble Bundle offering keys for games on these services would indicate the opposite. To prove there is a monopoly, you would first have to provide evidence that another company has attempted to offer a comparable service and was denied the right to. And if you think 3rd-parties weren't interested in keeping Sony, Microsoft and Sony's cut of digital sales for themselves, then you can't really argue that you have "reality" on your side when you're talking about companies who have done everything humanly imaginable to increase their bottom lines.
As I said in the post you quoted that you clipped out, neither of us have the means to prove our points as valid at this time. I thought the point above went without saying, I was clearly mistaken.
And the law can't be so toothless when Apple, the example I mentioned, already lost one antitrust lawsuit over ebooks and is
open and exposed to another over their App Store, which makes it an awfully perfect comparison to the point at hand. If there were collusion as you suggest, there's plenty of precedent in this arena to bring it before a court.
If we want to have a purely academic or legal discussion we can certainly do that. However, where the rubber meets the road, PSN and XBL are working market monopolies (or at best a collusional duopoly), with regards to pricing. There is *no other way* to (officially) acquire digital products through either platform. There's no need for conspiracy theories... This is a matter of common knowledge and operational fact for both companies.
Re: Nintendo, they're in their own alternate market (some might say alternate reality) for the most part, with very little if any software crossover or online/digital presence to speak of. Using Ninty as a way to bolster your argument that "a duopoly doesn't exist" because Nintendo gives people a "third choice" is laughable. Most (nearly all) 3rd party games aren't available on their platform and none of their games are available on either of the other two platforms. Ninty has virtually zero impact on either Sony's or Microsoft's online market or bottom line. That's an even weaker argument than trying to claim there is actual digital price competition between MS and Sony.
As an aside, we might be able to argue that hardware sales/user base and related physical game "attachment numbers" may have an up front effect on the gaming population pool's available disposable income to make digital purchases over time within each ecosystem. But that's a pretty flimsy argument for the existence of "price competition". Rather, that's more of an aggregate metric that could be used to gauge a publisher's willingness to allow a particular game to go on sale on one platform but not the other due to one ecosystem having less customers making digital purchases.
With that in mind, only a tiny minority/niche market of gamers/families who own both (or all 3) consoles per gen even have a *choice* where to buy their games digitally. And, contrary to your claim, most families can't just go out and buy a competitor's hardware because, for example, "game X" is $5 cheaper during a seasonal sale. That's ridiculous. IRL, the vast majority of gamers can't choose to buy a game from either marketplace - they're "stuck" with one. You're engaging in magical thinking here, as if you actually believe these platforms offer consumers products within a perfectly competitive, perfectly fluid market, which is just nonsense because they all run walled gardens.
Re: The Sherman Anti-Trust Act, again you demonstrate a fine academic grasp without acknowledging that IRL *barrier to entry* performs the relevant function. The cost of entering the console gaming business (or, say, competing with Apple's music biz) is prohibitive. Neither Sony nor Microsoft need actively / "artificially" prevent other players from entering to, indeed, have de facto monopoly/duopoly *pricing* power. Neither company need act like a "villain" for a form of price collusion to be true. Its called market momentum and it's very real.
Now, if we want to shift this discussion towards whether or not either company could be *proven* in a court of law to have violated the Sherman Anti-Trust Act then, yes, the burden of proof is raised. But, as much or more than the facts, politics and hefty bags of lobbyist cash play a huge role in pushing (or *pushing aside*) actual litigation (as an aside, I suggest you take a closer look at the Apple case as the current and likely end result is much ado about nothing. The fact remains that if you want to find anti-trust action with any teeth whatsoever since the late 70s you have to look to Europe, because it ain't happenin here).
Though it'd be naive in the extreme to assume either company hasn't conducted themselves in ways that, at the very least, openly flirt with illegality at times (Microsoft has infamously been under the microscope for decades for how they've conducted themselves in a monopolistic manner with regards to the OS division, with zero actual ramifications in the States)...we're not talking about the likelihood of legal battles in a highly politically contentious environment where who is pursued has more to do with whether or not they play ball than if they're breaking the law. What we're talking about is *effective monopoly* power over game pricing by limiting who has access to game keys. Your point about Humble Bundle keys seemingly makes no sense and is diametrically opposed to the your own argument. Its simple: If we have official 3rd party avenues to acquire game codes on either platform then *that* opens up competition. It's at least part of the reason why Steam is a superior platform for digital sales from the perspective of the consumer...because Steam hasn't locked out other ways of buying products on its service...and *this* is what creates actual price competition.