One point of proof-of-work is to allow chain continuity in the event of a network split. You can easily tell which chain has the most work sacrificed, but if a

One point of proof-of-work is to allow chain continuity in the event of a network split. You can easily tell which chain has the most work sacrificed, but if a netsplit occurs with a proof-of-stake protocol, it is vulnerable to a Sybil attack hijacking consensus with, say, a botnet.