That's not entirely correct. If 51% of the network is overtaken, and blocks are mined that aren't considered valid, that 51% will have hard-forked the network.

That's not entirely correct. If 51% of the network is overtaken, and blocks are mined that aren't considered valid, that 51% will have hard-forked the network. The 49% chain will likely maintain its value- since the new blocks won't be accepted by the rest of the network- and the majority fork will die. This is the whole "longest chain" versus "longest valid chain" distinction. A Sybil attack can censor the Bitcoin blockchain, but 51% doesn't gain full control of the network.