But Play Services and related libraries for analytics or ads have a documented API. Isn't it easier either to write an implementation of that API or put Play Se

But Play Services and related libraries for analytics or ads have a documented API. Isn't it easier either to write an implementation of that API or put Play Services into a safe sandbox rather than find time to write or port thousands of apps (that would require millions of man-hours) you could take for free from Android ecosystem? Also, there are open source apps like Calendar, Gallery, Camera or Music that don't have ads and don't need proprietary components that you could use for free.