Eventually you reach the limits of thermodynamics then you buy because they won't be anything faster coming out later.

22 nm — 2012
14 nm — 2013
10 nm — est. 2015
7 nm — est. 2017
5 nm — est. 2019

Not sure how many bitcoins will be left by 2019 though? 21m - 17m (2019 est.) =  4m?