You forgot that we aren't talking about some abandoned coins in a P2PK output. We are also talking about a much bigger amount of bitcoin (in total) in reused addresses, like a lot of the addresses in the bitcoin rich-list.
And in those cases, where addresses are being constantly reused, then almost all of those users will be able to move their coins to whatever quantum resistant algorithm we end up with, which will probably be in place years before the coins on reused addresses are at any meaningful risk.
The decision also won't be unilateral, whatever the decision may be. It will be a fork that like any other fork requires support from the majority.
I meant unilateral in respect to the owner of the coins. The majority shouldn't get to decide what to do with the coins belonging to someone else, even if we think those coins have been lost or abandoned.