How do you differentiate coins in lost wallets from coins that just haven't been spent yet by their anonymous owners?
Answer: you can't.
I know. That's why (for this lost wallet case) charging the demurrage when a transaction happens or after 17 years instead of charging it each block cannot be an advantage.
I think we just want them to prove they have the coins. In order to avoid the demurrage charge, they must show control of the coins by moving them. I don't think that's unreasonable to require every 20 years to avoid that charge.