First you said a mobile app developer doesn't have to worry about various screen sizes. But Android devices come in a ton of sizes and form factors. iPad apps are windowed and resizable, and a native Mac app runs on a system that connects to a monitor of arbitrary size.
So give that point was completely bunk, you changed it to something about the total number of internet connected devices. I have absolutely no clue why that is relevant, and you haven't explained why it is either.