Optional migration at spend-time only works if adoption is near-universal. Otherwise, untouched UTXOs become the weakest link. Either you break fungibility by confiscating them, or you risk collapse by leaving them vulnerable. That’s why the real defense is incentives + market consensus, not just praying quantum away.
Discussion
right. those are very confusing sentences, but sure. either everyone moves their coins into post-quantum scripts or q-day can still wipe the market out. hence my earlier points.