Sure it won't double forever, unfortunately it doesn't have to in order to destroy the profitability of most ASICs for the foreseeable future. 

For all intents and purposes a doubling of hashrate for 6 months with tapering of growth after that and doubling forever gives us roughly the same profitability calculation for this scenario.

In fact 100% per month for 3-6 months might be optimistic considering thegenesisblock has us at 126% for last 30 days before a single 28nm ASIC has shipped.  Then again it seems so many of these manufacturers have a hard time with timely delivery so nothing is certain but I'd be happy if we only stick to 100% / month for the short term.