This is the problem.

There's the obvious problem around apps and protocols. If you've been using them for a long time, your apps, their connectivity to each other, personal data etc is likely to be locked into Apple services & devices, or Google services & devices, in a way that is difficult or even impossible to move over (I know people with data they can only access now if they buy a license to apps they can no longer run).

More subtly, for example there was a recent article about how people in some places experience a social penalty if they aren't using iOS, because the difference is visible to others in terms of protocols used for instant messaging.

In my view, the great difficulty of switching and the penalties of switching mean that both Apple and Android ecosystems are, more or less, separate marketplaces, and each one has a strongly dominant player that should be subject to anti-trust measures.

Basically if you build a platform ecosystem, expressly design to lock-in users especially in ways that hugely affect their lives, that comes with responsibilities you don't have in a free and competitive marketplace where your users can switch with low friction.

Why isn't "the world" the smartphone market where there are plenty of options?

For a more meatspace example let's say you own a car with full self-driving capabilities. Now, the manufacturer of that car also runs a chain of grocery stores. It would be problematic if the car refused to drive too a different store. "If you've already to decided to buy a Walmart car then Walmart is the only store that you can go to."

At a certain point in debates like this everyone ends up over-examining analogies to a point where it isn't useful. To be a useful parallel you'd need a world where you can only buy two types of car and each one only lets you go to one type of shopping mall, or something like that. My point is, that isn't how either the real world works or how smartphone app stores work, so it's a debate over nothing.

There are two options, and one of them is provided free of charge by the world's largest advertising company.

The state of the smartphone OS market is so wildly different from the general-merchandise retail market as to render any comparisons meaningless.

So you can compare this to if you could only buy things at two different stores, each store has a hefty membership fee, and you were effectively locked into your decision for at least a year at a time.

"the definition of world is too narrow" is a pretty weak argument.

Such as Coca-cola (who even has to stock their own inventory), and those shops and restaurants that are at the front of every superstore location.

It's not like people have to choose whether to exclusively shop at Walmart or Target, like they do with phones. Reasonable people don't own both an iPhone and an Android.