I guess that at that point bitcoin would not exist anymore, that's it. You are mentioning bitcoin but it's the same with anything else: what happens if we extract all the oil available on our planet? It's done, no more oil. What happens if we kill all the elephants? No more elephants. And so on. Anyway a scenario like the one you are mentioning is extremely unluckily to happen, why would people choose to deliberately get rid of something that has a high value?