https://medium.com/@sashagnip/how-many-bitcoins-are-vulnerable-to-a-hypothetical-quantum-attack-3e59e4172e8Quote:
"as of 2018 June 4: 19% addresses (4,242,958 of 22,275,753) that hold 36% bitcoins (6,080,090 of 17,072,361) reveal their public keys."
Other bitcoin holders should be worried too in case if 6,000,000 bitcoins will be sold on exchanges.
The development of quantum computers is very slow so we have more time than that to implement algorithms that are resistant to quantum computers, so you are worrying for something that is not going to happen, it is very easy to think that something can destroy bitcoin but 10 years have passed and governments have been unable to find anything that comes close to that.
Don't worry, quantum-resistant algorithms are already developed. Bitcoin Post-Quantum fork serves the purpose of conducting an experiment about using them in Bitcoin. When it becomes necessary, these algorithms can be embedded in Bitcoin.
i'm not overly worried about the required changes. what worries me is how disruptive a post-quantum fork might be if we wait until after bitcoin's signature algorithm is broken. i'm not sure how a rollback would work in that case, and coordinating an emergency fork isn't easy. surely there would be major losses and the market would severely crash.
so i'm hoping we deal with this soon rather than rolling the dice and waiting until 2024 or 2027.