If you look back on, for example, video game development you can see the problems that can occur without such walled gardens. The home video game market boomed in the late 70s and early 80s, with families buying new games like hotcakes. A lot of game makers jumped on the bandwagon and pumped the market full of low quality games. Whereas in previous years the total number of games for the Atari 2600, for example, had been in the low dozens in 1982/83 this number ballooned to hundreds. Consumers could no longer have much confidence in which games to buy and so they stopped buying, leading to a massive crash of the video game industry in the US that lasted until Nintendo came along with its own walled garden approach.
However, the video game industry has matured since then, and walled gardens are no longer very helpful (there are far more than sufficient resources these days to determine which games to buy and which to avoid based on individual preferences).
As the mobile app market continues to mature it will strain against its walled garden confines more and more. Increasingly such hand-holding is less necessary and more and more restrictive. Apple has a choice to recognize that the market is changing and to adapt or to ignore the changes and pretend as though it's still 2008 while the world passes them by.