An alternative market would create pressure on Apple. If Netflix tells the App store to go screw themselves and lists on a different store, then the App Store gets that much less traffic and either has to compete or lose out.
Netflix does not need to go to a different store. They're influential enough to negotiate. Apple would cut them the discount, like now, and they wouldn't. The other store will not have enough users to make the reduced fees worth ditching the app store.
If there was enough competition, most of the app store profits would dissipate. That's what competition does. Users wouldn't pay 20-30%'extra to have apple's stamp, prices would fall and the business model is gone.
No little tickbox rule will make a competitive market in app stores. These markets are structured by nature. That's why platforms are so powerful.
Highly motivated users like kids who want to install Fortnite on their phones? That would give an "iOS Epic Store" a foot in the door, and while I'm not a huge fan of Epic, I think that would at least introduce some competition for Apple.
I don't see Netflix pulling out of the App Store if there alternatives. Right now, this is the only thing they can do to retaliate.
This doesn't really match what's happened with the PC gaming marketplace. Facebook pays for exclusives to the Oculus Store, Epic (whose lawsuit brought this e-mail to light) pays for exclusives on their store. A fair few major developers only release through their own store. Cases that buck this trend (like Halo releasing on Steam) tends to be news rather than typical.
The increasing trend of app stores on PC moving towards being silos is very worrying and harmful to consumers, but it's still a great deal better than what happens in competitive device-specific markets. Unless people like buying 3 separate gaming devices to get access to every console game that's released.
That many apps are available for both Android and iOS really has more to do with a lack of competition and a set of consumer expectations about availability than it has to do with platform management. There was a period where most PC games only released on Steam, too. But I personally enjoy being able to opt out of that store/DRM in favor of stores like Itch/GoG without losing access to every single new game that gets released. And I think my consumer choices have generally increased with the addition of newer storefronts that offer me different features that Steam refuses to offer.
It's easy to talk about the harms of fragmentation when you're happy with the status quo. But nobody who grew up protesting Steam's DRM is unhappy that more PC stores exist now, even if the market is more fragmented. And I similarly suspect that the people arguing that more choices on the iPhone would be bad for consumers are people who are happy with the app selection already, they're probably not the people who are frustrated that apps like NewPipe, Firefox, or emulators can only be accessed by moving to a completely separate ecosystem and buying a new device.