I absolutely do not want third party app stores or other payment options in apps. I don’t want to give my payment info to every app/store under the sun and risk potential data leaks where my credit card number gets stolen. I trust Apple to keep my payment info secure. That’s why I don’t buy much online. I limit my purchasing to Amazon (since if they were hacked everyone would be screwed) and websites that take Apple Pay or PayPal (which is a pain to use). I don’t give my credit card numbers out anywhere else.
If Apple could require apps to provide both Apple’s and 3rd party IAP I guess that would be the least bad of all options. I doubt Apple would be allowed to do that though - most courts would probably see this as an abuse of their market position.
I’m mostly worried about apps that only support 3rd party IAP or are walled behind third party app stores. While it has been pointed many times out that I can just ignore those apps, what happens when there is one that I actually need (not just want)?
As far as allowing third party app stores, I could potentially see larger devs like say Google forcing users to install their App Store to get their apps (based off what they did with Windows Phone this doesn't seem that far off). Or what if some government like Russia forced all apps installed in their country to go through their App Store? Who knows what kind of surveillance could be injected this way.
As a consumer I already have options for what payment method actually backs Apple's IAP - whether that be a credit card (Visa, MasterCard, Discover, etc.), PayPal via linked AppleID account, or cash via purchased gift cards. This isn't really about consumers, this is about devs that don't want to give Apple a cut of the payments. I highly doubt prices would be lower with 3rd party IAP - Apple already lowered their commission from 30% to 15% for most apps and I'm not aware of any that have lowered their prices to pass the savings onto the consumer.
Physical stores already limit what payment methods consumers can use. Visa and MasterCard are accepted pretty much everywhere though Costco only accepts Visa. I've already been to places that won't accept my Discover card and American Express has an even lower acceptance rate than Discover. Some stores don’t even accept credit cards. There are even stores that will give you discounts for using their store loyalty/credit card. Should they be forced to accept all credit cards?
Edit: This will benefit large devs the most since they will have the infrastructure to host app stores (think Microsoft, Google, Amazon, Epic) and then they can pocket 100% of their IAP themselves. This won't help small devs nearly as much - they would have to partner with other app stores to be hosted. I can even envision certain apps being signed to exclusive deals with certain stores (similar to how streaming services get their exclusive content). What happens if an app moves stores? Do I need to re-download it again to continue to get updates? Will I have to re-buy my IAP since that was tied to the original store?