>Do you set a threshold day

yeah pretty much.

>What do you do with old coins? Satoshi's one for example? Or lost coins that nobody has the key for?

If those coins hasn't been touched for decades, despite widespread announcements of pre-quantum cryptography (presumably it wouldn't happen overnight), it's safe to say that nobody is going to claim them.

Dont treat any address differently

You wont know that, you will provide the tools and also new accounts will have assurance and regenerate confidence in the system

So we have moved from concluding that bitcoin use is an irreparably flawed concept to a method of maintaining viability of the concept