I agree price is a factor, but there are a lot of GPU's out there with bored users, that don't need a lot of incentive to just point unused cycles at the network.  Users like me for example. Widespread media attention (the hacking of the 25k coins, the acceptance of BTC by wikileaks, MtGox articles, etc) will only bring new users onboard who don't need to invest much if any capital to continually double the hashing power of the network.  I am not saying it is bad, but I completely disagree that it is unrealistic.