Apple

I don’t see that happening just because of the technological hurdles there will likely be to get the third party store.

Android has had this ability since its very early days but most people still stick with the default Google Play Store unless they got something like an Amazon specific device.

Because of two reasons:

• most users don’t know other storefronts exist
• even if they did know, most don’t want to go through the hurdles of figuring out how to download the new store, get it set up correctly, disable relevant security that allows third party installation, and set up an account for another storefront

This is further unlikely to happen because Android has given Amazon and other manufacturers the ability to create their own Android devices whereas Apple has not and likely never will. You will likely never be able to buy an iPhone that has a custom ROM from Microsoft, for example.