if the profit is the same, as you imply, then people will try to get in on the game - competition - which forces prices down.
Until costs exceed returns, then some people will get out of the game, difficulty goes back down, and it's once again more profitable. Are we talking past each other?
OK, so the profitability goes up and down as people go in and out. But what is the average profitability once you smoothe out the swings?
The original point made was that the average profit will tend toward zero.
If the average profit is above zero it must mean someone else is bearing the costs, like, your network provider.