Well, the main thing is..

Higher profit per 1Ghash = more miners. (higher security)

Lower profit per 1Ghash = less miners. (lower security)

Every miner has a certain profitability they're trying to reach, otherwise they will drop out.
Sort of seeing this already, even at the current profitability. Network growth has basically stalled.

In the future, I can sort of see ultra high efficiencies in mining, with huge companies supporting the entire network, but that's years away. Bitcoin would have had to reach some sort of ultra-stability and high trust for that to happen.