By that logic a supermarket would sell only 33 items.

"Try a different smaller form factor every couple of years" is a reasonable strategy. First Mini, then Air. Supposedly they're gonna refresh the Air next year, we'll see. But even if they don't: we're well into the "keep phones 3 years or more" era of the smart phone. You don't need an exact match for every phone for everybody every year.

6 to 7 million units is a good number.