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.
More miners decreases the profit per miner. It's actually about the ratio between difficulty and value of BTC, and where it will eventually settle.
I argue that mining profitability will eventually match others similar markets, which means that at the current BTC valuation, difficulty would be somewhere around 5,000,000.
Currently, you are getting back more than twice what you pay in electricity, which should still be very lucrative to many. But you need a lot of hashing power to make meaningful money.
Sort of seeing this already, even at the current profitability. Network growth has basically stalled.
I think that has more to do with the DDoSing of the pools than miners actually dropping out.