Poor analogy. iOS does not have the market dominance that Windows/Microsoft has. In this case, iPhones represent about 15% of the global smartphone sales and I think that the OS (in the US) is a 60/40 iOS vs android split. There is a viable market. Developers, unwilling to pay the Apple fee, can switch to Android. If more apps are available on Android, that will shift the users away from iOS to Android.
When I see folks complain about this, I like ask "what do you think is a reasonable fee for Apple to charge?" Zero is not a realistic answer as Apple does incur costs to run the app store. Moreover, they're entitled to make a profit off the marketplace they created and support. So what's a reasonable percentage?