The moment it becomes impossible to get any money out of the system (back into fiat) that's the moment it becomes near worthless.
I don't think you understand how Bitcoin works.
If I am willing to sell $100 worth of goods for a Bitcoin, and I can safely assume that an equitable exchange is being offered by somebody else, then that sets the value. It isn't about getting dollars back out at the end.
Looking at it as a price-first-then-trade instead of trade-first-then-price valuation system is putting the cart before the horse, and has a negative impact on the actual utility of Bitcoins.
This assumes you have mass adoption, which Bitcoin does not. It has a black market for buying goods which hardly gives it tremendous value. There is a reason businesses, especially US businesses are not excepting Bitcoin. They know by the laws on the books already, that Bitcoin is illegal. No mass adoption, no value. So the only value you have, is what you can drum up in the black market.
The only reason the prices have reached +100...is because of speculative greed. Nothing more, nothing less.