Don't be so sure
Aol and others failed because the wall was obvious and their selection was much worse by comparison. Both the Android and iOS stores are already more mature, and more importantly so is the audience. Until something game changing comes along I expect the walled gardens to stay.
In fact walled gardens on the net have existed far longer than the free Internet.
Just because there is freedom on the web doesn't make it intrinsically superior to the walled gardens - as long as both meet users evolving needs.
Don't assume the free web will win just because you feel it should. The average user doesn't give a toss so long as their needs are met. The Internet just follows the same rules area the rest of our social institutions 5% lead and as long as they are content 95% follow.