If such a threat were to exist (it doesn't), that would be the incentive to move coins... not fear of lockout.

No one is forced to accept stolen coins, Lopp's argument is disingenuous. If you receive abandoned coins you suspect were stolen by QC you can burn them.

This is the not-so-quiet part of the proposal, fear of dilution by reintroducing abandoned coins onto the market. Lopp undermines his own argument, they're not afraid people will be forced to accept coins, but that they will happily accept them.

This is not a technical BIP, it's a vanity one trying to impose a view on the market. The very market fear driving it is also what undermines it, you can't fight the market. Someone stealing a satoshi amount of coins will also buy the hashpower to confirm the tx.

A functional QC would effectively be a winner-take-all coins, candidates in the running to have it already have access to a material number of coins (Google, Microsoft have countless passwords emails 2FA to custodial platforms and password managers).

A QC could also sign for any software distribution, enabling untold backdoors into systems. There's no change to Bitcoin that can protect Bitcoin from a (fake) QC threat because Bitcoin is not the only link in the chain.