If you don’t also drop wallets with compromised signatures at some point after introducing secure signatures (effectively editing the ledger) they will be up for grabs.
Absent a functional ledger rewrite I expect there would be some window where miners with access to CRQCs switch their focus over to exclusively mining blocks of transactions transferring coins from insecure wallets to secure wallets under their own control. Is there actually interest in living in the world where the first person with both a CRQC and a mining farm gets to claim all of the stranded bitcoins for themselves?
Absent a functional ledger rewrite I expect there would be some window where miners with access to CRQCs switch their focus over to exclusively mining blocks of transactions transferring coins from insecure wallets to secure wallets under their own control. Is there actually interest in living in the world where the first person with both a CRQC and a mining farm gets to claim all of the stranded bitcoins for themselves?