In that case Bitcoin might be caught in a "big freeze". Bitcoin takes about 2000 blocks to adjust difficulty, so if it becomes worthless overnight, the amount o

In that case Bitcoin might be caught in a "big freeze". Bitcoin takes about 2000 blocks to adjust difficulty, so if it becomes worthless overnight, the amount of hash power could drop enough that no new blocks will ever be found, and without any new blocks the difficulty will never adjust. So one player would need to buy the vast majority of all miners, and even then they would only be capable of doing a double spend attack on a dead blockchain.