If the majority of work is controlled by honest nodes then the chain is safe. Not done by honest nodes, controlled by honest nodes.
With PoW an attacker might produce new miners and gain control of the network.
With PoS unless enough people sell to him their money, a "new" attacker just can't do anything.
(not implying there might not be other attack vectors)
Yes.... Also, other attack vectors and unexpected vulnerabilities are constantly turning up as algorithms and systems evolve. That is one reason for the 'newer' coins being more in demand (in general), because there is a constant search for 'fixes'.