If you look at the 6 month chart it's fairly obvious there's something unsustainable going on.
Those are your two options. Grow until the Bitcoin economy is worth some 10 Billion at least, or it'll be evidently dead and close to 0.
Its much more unlikely Bitcoins will stay around $20 - $60 for the next 5 years. (Although still possible)
Using the only scale we have available (history) $40+ per BTC is VASTLY overpriced considering the previous historical fair market value.