It's not the actual selling of the device the problem, it's the way you are locked in the walled garden. Let's say, you don't like the next iPhone. You want something different. Well, tough shit, you will need to find a new way to store all your photos and files since you won't be able to use those either on any other device. Same goes for anything else you might have in your iCloud.
Now, I'm not saying that you can't use other services or anything, all I'm saying is that if you put all your eggs in one basket, it becomes hard(not impossible, usually, just harder) to then move them away.
