Creating an identity on nostr is basically free. So imagine an attacker creates a whole new network of nibs following each other, exactly replicating the structure of the real network of npubs, same structure.

The two graphs are isomorfic, so any graph analysis will yield the same result. Hence, if you don't have a pov (e.g brand new npubs that don't follow anyone) you can't distinguish between the two, hence my focus on PoW for this case.