If the price of bitcoins seriously shoots up 1000 fold in the next few years, those 50BTC or 25BTC rewards will be worth a lot of money! At $10,000/BTC in 3 years, each reward would be worth $250,000. In this case the mining won't be limited by the competitive market, it will be limited by the supply of graphics boards and the amount of power the electricity companies will ration you. This is the scenario I'm pointing to. It's the scenario that's implied when people say Bitcoin is scalable to cope with a significant %age of world GDP.