It wouldn't, at least not in a useful way. At that point there's two major issues: hashing becomes impossibly slow, network partition aka forking the chain, and

It wouldn't, at least not in a useful way. At that point there's two major issues: hashing becomes impossibly slow, network partition aka forking the chain, and validating transactions. Just distributing a new block becomes a huge task in and of itself. Also before a transaction can be included our hypothetical human miner has to check to ensure that the inputs/source address has a corresponding output/target address earlier in the chain. That step could require scanning the whole 15 GB blockchain to confirm that a transaction is valid.