The only issue with a hash cash is there’s no way to know whether the user’s browser is the one who computed said proof of work, or has delegated it to a differ

The only issue with a hash cash is there’s no way to know whether the user’s browser is the one who computed said proof of work, or has delegated it to a different system and is simply relaying its results. At scale, you’d end up with a large botnet that receives proof of work tokens to solve for the scraping network to use.