So, okay. Apple/Valve/Google/GoG/etc run digital storefronts. I believe that the industry standard today is a 30% cut for the vendor.
So one option is what the vendors would like to do, which is that all purchases should go through them.
Apple is probably a special case, because Apple both owns the platform and doesn't allow for alternate app stores. If you want to sell software to iOS users, you must go through Apple.
Google also owns a platform. Google does permit for alternate app stores, but doesn't default to adding any. In practice, I suspect that the defaults are very powerful -- only a small percentage of users will probably add a non-default app store -- but people who do want to can add alternate app stores.
Then you've got Valve and GoG. They don't own any platforms (well, okay, I guess Valve does in the form of the Steam Boxes and the Steam Deck, but they're a small portion of the actual consumers using the digital store; doesn't really work the same way). Basically, the bulk of people using these needed to "opt in". In the case of Windows users, I guess the default is the Microsoft Store, which I've no familiarity with. Steam is packaged in Debian and I assume most Linux distros, but not installed by default on any distro that I'm aware of.
I don't know what policies for non-Apple platforms are on in-app purchases, whether the digital storefront needs to get a cut. I suspect that at least for Steam, they likely do, just because Bethesda sells credits for their in-app purchases on Steam, but I haven't read up on that. I dunno about this "steering" stuff. I know that Valve has cut deals to let other people "link" purchases on other stores to Steam, but I don't think that they're obliged to do that -- they're doing it because they think that it's worthwhile to miss out on some sales revenue if they can help build the userbase that they can put a store in front of.
So there are varying levels of ways in which the vendor is privileged by the platform to which they are selling software, from "nearly guaranteed a monopoly on selling for that platform" to "no benefit at all".
But all of them rely on a commission from the software that's being sold through them.
I'm onboard with the idea of them getting a percentage take. I dunno if 30% is the right number -- my own gut feeling is that that's high -- but they're legitimately providing services. And for at least Steam and GoG, they're competing against each other (and I guess Microsoft Store, for some platforms), so that's even in the presence of competition.
Another option is to have no constraint on a cut for in-app sales (or "steering", which I think is probably functionally pretty similar). That is, have app stores obligated to permit listings even if apps direct someone to use another purchasing method, and allow that method to not let the app stores have a cut.
The problem with this route is that the end game would, to me, seem to be that publishers simply do a $0 "demo" listing to market their product on a digital store, and then have someone "pay to purchase" full game content in-game or via another payment mechanism. The app store operator isn't going to get any cut then. Like, that's not really a viable route for them.
Hmm.
I think that a viable solution should have the following characteristics:
If someone's buying the product through the app store, then the app store vendor should be able to get a percentage take of that revenue. That's basically how brick-and-mortar retail works. The publisher shouldn't be able to just route around the app store's payment mechanism to avoid that. It's not clear to me that what the EU presently is asking for is compatible with that.
The app store should not be able to have a monopoly on a platform. I don't think that there's any justifiable business case for letting companies set up that kind of vertical monopoly. While Apple doesn't lose money on phones, console vendors do lose money on hardware and run a market that they have a monopoly over, so this constraint would break their existing business model. I'm okay with breaking that, though I can see maybe phasing it out over a generation, since they made their existing investments based on the expectation that they could do so.
I'd like buying from one app store not to force a user to be unable to use that purchase with services from another app store in the future. Maybe one way to deal with this would be to mandate that app store permit "transfers" of ownership, but be permitted to charge a percentage fee for that transfer. Like, if I buy something from GoG, then Valve has to let me transfer that purchase to Steam. That still lets app stores compete on and pay for things like services (e.g. Valve provides Proton and Steam Input and stuff like that), and lets them compete on price, but doesn't let them lock a user in permanently just because they bought a product on that service. It also has some other nice perks, like dealing with app stores that go out-of-business -- and while no big ones have gone under, that is going to happen in the future at some point.