If malicious actors with access to quantum computers steal coins, even with various blockchain analysis tools available, none of them perfectly prevents launderings, and attackers would likely succeed in laundering the stolen funds.
This could lead to Bitcoin losing public trust - creating a lose-lose situation for both society and the Bitcoin community as a whole.
Bitcoin's security would be impacted and people would lose trust in the network. However, the solution proposed in this BIP doesn't increase trust in Bitcoin either. It doesn't look trustworthy if a decision is made that your coins will become non-spendable in the future.
It's not good either way.