so its not profitable to buy antminer s2
It costs 2167 $ right now without discount coupon @ bitmaintech.
You saying that in a few months earnings will drop from 26$ / day to 11$ / day or less
So whats the point of buying a 2167$ machine?
The point is the company is selling you a machine that will mine $26 a day! Think of it!
After 3 months you'll be getting pennies a day and be several hundred in the hole.
The seller of the ASIC now has $2167 which they could have used to buy more BTC than you mine.