You have a woefully incorrect guess of how much ads make. Woefully.
I think you need to read this.
So in terms of returns, Android is sustainable. However, in relative terms the value created leaves much to be desired. Whereas Android generates $1.70/device/year and thus an Android device with a two year life generates about $3.5 to Google over its life, Apple obtained $576.3 for each iOS device it sold in 2011[4]. The economics of Android are nothing like the economics of iOS.