But the emphasis is on how quickly would it be fixed and be ready to be deployed. In the mealtime, the value would drop. Not to 0 or anywhere close. We have seen huge drops recently. I think 30-40% in a day during the first wave of the COVID-19 pandemic.
The value would drop, sure, but I honestly don't think it would drop that significantly in such a hypothetical scenario. First of all, if the entire network cannot make transactions, then the only bitcoin which can be sold is that which is already on centralized exchanges. Second of all, a fix to such a critical bug would likely be deployed within a few hours. The value overflow incident, for example, was patched within around 90 minutes of the bug being exploited, with far fewer people working on the code then there is now. Third, if the price was falling solely due to a bug which everyone knew was going to be fixed within a few hours, then many would see that as a great time to buy the dip.
Quantum computers could destroy Bitcoin completely
Why? What's stopping us forking to a quantum-resistant algorithm?