> You buy different stuff, copy your data across and sell the original stuff.

You buy a different house, move your stuff across and sell the original house. How is it a crappy analogy?

The issue is that the cost of moving removes your choice from individual decisions because they all have to be made together. If you want iMessage then you have to sell your Android and get an iPhone. If you want F-Droid then you have to sell your iPhone and get an Android. What if you want both? This isn't because the free software community would be unwilling to set up a store/repository for iOS, it isn't because no Android messaging app would be willing to interoperate with iMessage, it's because you're locked in to one platform or the other at any given time and have to make all your choices together.

Someone who wants to provide an app store that charges lower fees would have to convince everyone to switch to their platform instead of only convincing people to switch to their store.

The reason they make it that way instead of being able to choose what you run on your device independent of the kind of device is in order to lock you in.