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the physics is cool, but the engineering needed to scale may still well be impossible in the physical world. bitcoin *cannot* respond to claims that unicorns exist with protocol change
We may never have a CRQC that's a real but unlikely possibility. Let's say you believe in your heart of hearts that CRQCs are impossible. Algorithm agility is still critical to the future of Bitcoin in such a world.
To quote from
Guidelines for Cryptographic Algorithm Agility and Selecting Mandatory-to-Implement Algorithms (RFC 7596)"Cryptographic algorithms age; they become weaker with time. As new cryptanalysis techniques are developed and computing capabilities improve, the work required to break a particular cryptographic algorithm will reduce, making an attack on the algorithm more feasible for more attackers. While it is unknown how cryptoanalytic attacks will evolve, it is certain that they will get better."
...
Protocol designers need to assume that advances in computing power or advances in cryptoanalytic techniques will eventually make any algorithm obsolete."
A CRQC is one of many threats to the cryptography used in Bitcoin signatures. If we want Bitcoin to be a secure store of value over at least one human lifetime, then algorithm agility is a must. Part of that security is that your coins don't get stolen due to cryptographic weaknesses, part of that security is that know your coins are unlikely to get stolen, i.e. epistemological problem.