You speak of false equivalences after equating Android with iOS... There are countless different Android distros. Most Android phones can be unlocked easily and/or are sold unlocked in certain markets. All software components are swappable. The OS can be run entirely Google-free, if you want. The Play Store is default, but optional. Sure, the mainstream hardware manufacturers sell preinstalled Android distros that require a bit of menu diving in order to enable free software installation, but there's little that's stopping you, as a customer.

I do not know of a functionality not available for development on a open source Mobile OS.

I agree that it costs time, effort and money to develop each functionality. I fail to see the choice between only iOS and only-Android. It is not mandated to choose only between these two.