How would you do that? You can still increase the fixed difficulty by hand.
Exponential fit. There is a calculator online:
http://www.taters.net/btcgc.phpAlso there seems to be a correlation to some extend between difficulty and price which partly compensates the increase in difficulty:
As long as mining is marginally profitable, difficulty will increase even with constant BTC prices. And a claim of increase in BTC price is a pure speculation.