This is the fear that someone will somehow obtain control of Satoshi era 'lost' coins.

This isn't about any specific person losing control of their UTxOs to a quantum attack.

Initially we'll have a soft fork for a new type of address that is somehow quantum resistant. I don't think that is too contraversial although I won't be in a rush to move my UTxOs.

You'll then have a hard fork pushed that nullifies UTxOs on all legacy addresses.

This is the real threat. Banning addresses of a certain type. This is a form of censorship pushed under the guise of 'for your own safety'. Once we go down that path of banning addresses, we set a very dangerous precedent. Bitcoin would be no longer censorship resistant & decentralised.

We then have 2 Bitcoins, 1 captured & 1 free. The real question is which one would people value? The captured Bitcoin that doesn't have all valid UTxOs available or the free Bitcoin that potentially has Satoshi's coins being spent.

I think the ultimate decider will be whether those coins move. While those coins stay dormant, the uncensored Bitcoin will be the winner. As soon as those coins move, the need for a hard fork can be easily justified.

Pre-emptively hard forking to ban addresses should be fought with everything we have. Bitcoin is about individuals asserting their sovereignty. Nobody can force you to run their code.