At the point at which half of world economic activity was denominated in bitcoin is about where the exponential increase should begin to slow and eventually level out.
Or it may not slow down. If half of all economic activity is done in bitcoins, the value of all fiat halves. It'll trigger the positive feedback loop, "the fiat run". So it'll be an L-curve, rather than an S-curve. Fiat will not wither, it'go down in flames
P.S. As a side-effect we can expect a lot of people suddenly left with useless fiat. Angry mobs lynching early adapters.
