The specter of quantum computer systems is principally that they’d break Bitcoin’s elliptic curve cryptography, which means they may derive a non-public key from a public key. There’s additionally the potential risk to mining, however I consider that is far much less understood (it actually is by me), so the next applies mainly to the elliptic curve cryptography.
The overall answer to the quantum risk is to introduce a post-quantum (PQC) signature scheme, which could possibly be achieved with a gentle fork. Then, everybody must migrate their funds to the PQC scheme in time earlier than the quantum risk materialized.
The query then turns into what to do about cash (UTXOs) locked with susceptible public keys. One choice is to do nothing and let the cash ultimately be stolen by quantum attackers. Another choice is to make these cash unspendable with a gentle fork after a set deadline. But an alternative choice is to make these cash unspendable utilizing the susceptible signature scheme alone, and there are numerous option to go about that, however all of them could possibly be achieved with a gentle fork.
The one time we would want a tough fork is that if we might first wish to make the susceptible cash fully unspendable with a gentle fork, after which later wished to re-enable spending with a safe signature scheme. Except the specter of quantum computing materialized all of a sudden and unexpectedly I do not assume we might do this (and we would have larger issues anyway).
