they both seem to agree
Discussion
(if/when quantum computers become powerful enough to allow stealing bitcoins):
Pieter suggests the bitcoins that are still vulnerable are confiscated/locked/destroyed/made unspendable.
Parker suggests allowing the vulnerable bitcoins to be stolen.
Third option, there is no quantum threat yet it also poses the biggest attack vector on Bitcoin; one that induces action from psychological fear. Bitcoiners walking right into the trap, trusting centralized quantum computing, not verifying.
There is no second best.
Of course you let them be stolen. You can't lock up someone's bitcoin.