Here is a link to something that I feel is my optimistic view as respects network difficulty.
http://btcinvest.net/en/bitcoin-mining-profit-calculator.php?diff=249506922&dcosts=31200&diff_mincrease=35&blpbtc=25&dhsmhs=2200000&diff_mincreasedecrease=3&btcusd=140&dpowcon=2400&btcusd_mincrease=3&pcost=0.13&calcweeks=25&dleadtime=0&action=calc#
http://btcinvest.net/en/bitcoin-mining-profit-calculator.php?diff=249506922&dcosts=31200&diff_mincrease=35&blpbtc=25&dhsmhs=2200000&diff_mincreasedecrease=3&btcusd=140&dpowcon=2400&btcusd_mincrease=3&pcost=0.13&calcweeks=25&dleadtime=0&action=calc#
Out of curiosity, what makes you think the growth will start slowing down next difficulty retargeting?
It will start slowing down at some point, no one doubts that, but so far, its been speeding up (http://bitcoin.sipa.be/growth.png) and all the major roll outs have yet to happen.
More people will get the idea of how to invest into btc and see other opportunities open up.
Who knows. Maybe a public bitcoin ETF in the states. But then again. WHO KNOWS....
That is the risk of participating rather than just using BTC.