Your question is kind of like saying why won't the price of gold crash when the last ounce has been found. When something is in demand and there is limited quantities of it, why would it ever go down in price when no more can be discovered? These are basic laws of supply & demand. There is only so much Bitcoin out there and when the final coin is mined (and assuming there is still demand for it) the price will not crash, but instead go up because everyone will know all that's available is what's already out there. I think it all comes down to supply and demand. If there is lots of demand when the last coin is mined we could see it go up. If there is no demand for it when the last coin is mined then it could go down, or just not change at all..