If it happens that 128-bit entropy becomes vulnerable.
Bitcoin is not the only thing that's doomed to fail. The bank system, e-commerce, and communication activities will be affected too. However, the quantum computer that has the potential to do so will take at least 50 years to be developed, and I believe the government dont want to see that happen.
I wouldn't like to speculate how much time would be needed to develop a quantum computer that is strong enough to break modern encryption. 10 years ago, no one would have told you how far we would get with AI and look where we are now. Can you imagine how it will look 10 years from now?
Many industries would be affected. That is true. But I think the centralized banking industry would have an easier time adopting a quantum-resistant algorithm than the decentralized Bitcoin. God forbid that we will have five equally good solutions and we as a community would have to pick one. It's more likely that we would get two new forks and new splits in the community than a majority vote for one solution.
Yes, it is hard to speculate how much time or years it will take before the quantum computer that can break entropy, but it is easier for the decentralized Bitcoin ecosystem to adopt quantum resistant algorithm.
According to the information I once read by Quside, the platform that developed new generation quantum technologies for connectivity and efficient computation. There's already a team working on quantum cryptography (like code based cryptography, lattice based cryptography) and post-quantum cryptography, which are said to be classical algorithms to combat quantum decryption, so I don't see any reason why it will be hard to have the creation of quantum resistant algorithms in Bitcoin.