the problem is that markets are irrational. this is a direct result of the CB's creating moral hazard via pumping of fiat. they literally WANT us to speculate to drive risk assets higher. thats all they have at this point and its going to work. otherwise watch your USD's devalue. the pendulum swung up to 32, down to 2, and now...
This is why the Bitcoin market may be the most
rational one on the planet at the moment. Yes, it is affected
by capital flows from contemporary markets, but it reacts
the way a free and open market would - overall
, market participants learn and adopt mostly reasonable practices (nothing is perfectly efficient or rational).
The size of the market and the stock to flow ratio are the major factors to consider for the long-term. As the stock to flow ratio increases, fluctuations should stabilize without external flow as there will be reduced incentive for miners to dump newly-found coins. Any external flow (USD->BTC, etc) will cause exchange rate price movements, but larger pools of wealth tend to leave increasing traces of themselves in the smaller ones, just like fiat->gold - it's a gradual transition.
I would say general adoption of Bitcoin could easily bring the BTC/USD rate to the mid $60s along a gradual rise within four years (assuming no further QE... ha!). However, I doubt things will be very gradual. It's more likely than not there will be more exogenous shocks that cause volatile foreign exchange prices.