What if Santa Claus was real.
In all seriousness, it would never happen. Modern currencies are far too ingrained to disappear anytime soon, and most likely by the time that happened all of the BitCoins would be fully mined, and we'd probably be dead.
Yes, but I'm sure people don't enjoy the inflation that's caused by having worthless paper as money. If the amount of Bitcoins are really limited they would be a much better currency inflation-wise