Rewarding people for being on the network (mining) was a brilliant idea. The hope is that this creates a network stable enough that people begin trading in bitcoin. Then, once the reward for mining diminishes, people still want to be on the network because it is valuable for trading.

BUT, this means that mining profitability should not decrease before bitcoins become popular with lots of people for non-mining activities. It's possible that mining profitability will go down (lots of new rigs, reward eventually dropping to 25btc) before bitcoins are popular enough. What happens then? Lots of miners leave the network... What then?