I’ve previously stated one reason i’d be worse off. Since apps sold in those other stores would have to be actually able to RUN on ios, that would mean a weakling of the certificate/provisioning security model used to ensure that only apps that Apple has approved and which are unmodified from the version submitted by the developers to the App Store can run. Whatever the new mechanism is, the laws of cryptography tell us that this introduces a new attack vector.
Then imagine that Apple modifies the operating system to provide new features for developers who sell apps through apple’s App Store. As a result of these changes, apps sold by Joe’s crApp Store stop working. Now Joe sues Apple, and Apple gets gunshy about dropping support for out-of-date SDKs. So the OS slowly becomes decrepit and Android/Windows-like.
So there are two reasons right there why we’d be worse off.
Now, again, prove that somehow we’d be better off. It can’t be that prices will come down - after all, 85% of apps are free, and the rest are 10x cheaper than pre-App Store (and no different in price than on Android, where this “freedom” exists). It can’t be that we’d meaningfully have “more choices” of apps, because those who care about such things would be on Android anyway. So what possible plausible advantage does this bring other than introducing chaos?