Not if they can attack the network without anybody knowing. If an attacker secretly controls 51% of the coins (by, say, having three separate entities that each

Not if they can attack the network without anybody knowing. If an attacker secretly controls 51% of the coins (by, say, having three separate entities that each control 15-20%) and can manipulate the market in their favor by rejecting transactions that would hurt them, that's a big incentive.