Being able to watch a wallet sounds more of a bug than feature to me.

My point was, in practice, everyone is relying on a node to do that work. They are not verifying it for themselves. They have no clue how much bitcoin is out there for certain.

Regardless, there is no definitive way to prove what bitcoin is lost. You can never be sure sure if an address truly lost their keys or are just hodling. Something like 80% of bitcoin is illiquid. Even Satoshi's unmoved bitcoin could suddenly start moving. We don't know for certain what bitcoin is truly lost.