You're conflating having a single app store with having a consistent user experience. Mac OS didn't have an app store for most of its existence and it's doing fine.
Android is particularly bad in that regard, not just because of the ability to side-load alternative apps and app stores. The main issue is that OEMs heavily modify the OS for their devices, something that isn't an issue with Apple's business model.
I'm not so sure it's a conflation. Having a single app store makes for a more consistent user experience when moving from device to device.
You can tell people about apps and they know how and where to get them. See an app you like on another iPhone? You can probably install it on yours fairly easily. There's less confusion around the entire ecosystem because of its simplicity.
