A good point from the forum.
“Now PARSEC won’t be used in SAFE to let the entire network communicate and come into global consensus like for example Bitcoin does, instead PARSEC will be used by many small sections of the network to reach “local” consensus for the corresponding section of the network. The obvious attack vector here is to join one section with many nodes of your own to hijack that particular section. The idea is to make this attack infeasible by having the network occasionally relocate nodes (when they age/rank up), so an attacker doesn’t have control over in what sections of the network its nodes are located. If this idea works as well in practice as it does in theory, attacking a section is about as hard as attacking the entire network (all the sections).”
So proof-of-work is inherently resistant to sybil attacks while PARSEC on its own isn’t, so to tell the story properly the random but deterministic relocating of nodes needs to be included.