Great in theory, but there really isn't a good way to diversify if you're making apps. Yes there's Android, and some apps can make as much or more money on Android, but even if the app is making the same amount on both platforms, iOS kicking you out for no good reason could cut your income in half, which can put a company out of business almost as quickly as cutting off all of their money, if they've grown it based on what they believe to be a steady income stream.
Given the kind of draconian control apple has on its store, there is always a risk if you are part of the ecosystem. But earning half income is still better than none right? Besides now windows has app store too and a much bigger distribution. If you don't like Apple's terms and your other platforms are making enough money you always have an option of kissing goodbye to apple. My point is that diversification gives you options.
Both androind and Windows provide guidelines to port over your applications over. So I don't think this option exists only in practice.
