It also gives Apple a big lever to ensure the platform stays agile. For example, Apple was able to make 64 bit happen first because they knew what software coul

It also gives Apple a big lever to ensure the platform stays agile. For example, Apple was able to make 64 bit happen first because they knew what software could be installed (so they knew what the compatibility risk was), and because they controlled distribution. If there were another major app store with significant market share, Apple would find it more difficult to evolve the platform.