mostly android app earn income via ads , sales in google app store are really bad. no body is buying.

This is at the heart of the problem. Nitpicking at Gruber specific points is one thing, but the fact is that paid apps on the Android store are getting nowhere. Arguing that "this doesn't matter because free is the way to go" is a reasonable opinion to hold, but misses the point. Android developers don't get to make a choice about that whatever their opinion, while iOS developers do. The counter argument is that as these platforms mature and become more capable the apps will become more functional and sophisticated, with attached services and customers will be more willing to pay for them. iOS developers have a shot at making that future happen, unless things change Android devs don't.

Do you have any basis for your fact that "paid apps on the Android store are getting nowhere"?

I've seen a few things suggesting lower sales rates on Android, but since almost none of them adjusted for installed base, I'd be wary of drawing any strong conclusions. But even if they are correct in their inferences, they were still suggesting simply lower sales (e.g. a third or half iOS) not that no-one was buying apps at all. It's hardly a black and white picture, particularly with the introduction of iAds by Apple.