I agree that the margin for profit is small, but not profitable is not true.
Most miners has experienced both loss and gains, but that is properly not much different that any other business.

The reward for mining is self-adjusting, as long as there is a block reward to compete on, then will there be miners to race for the reward.
If the block reward exceed the cost in electricity and roi is impossible then will some miners (with lower hashrate = lower probability to find the block) shut down, and then will the difficulty go down, and then will roi be possible for others. The system will self-adjust to a level where there is a profit for the miners.
 
Even when all 21 mil BTC has been mined will there still be competition to claim the tx-fee's in the block.

The system (bitcoin) is on a ramp-up right now, so it is very difficult to make any scientific conclusions based the current situation, I believe a more holistic approach is needed.