You would have an interim period with some severe volatility for sure — the actual thing that would happen was like when Ghash.io did enable double spends against SatoshiDice. This stuff has already happened in Bitcoin before. It isn’t the end of the world, and certainly did not permanently eliminate the viability of Bitcoin. It caused a price drop of roughly 5% when it happened. It’s bad, but it is a temporary thing that is mitigated by requiring more confirmations.