PoS as well as PoW depend on the assumption that a majority (either in terms of value invested in hashing power or invested in tokens) of the network is following the rules. If you try to double spend in a PoW system you are betting your current electricity usage on a new block becoming the longest chain and in a PoS system you are betting the tokens themselves - from an economic point of view those two systems are indifferent (if your attempt doesn't succeed you have wasted either electricity or tokens). Collusion can happen in both cases. PoS systems introduce new (probably yet unsolved) problems but they aren't inferior from an economical point of view imho.

51% coalitions can also create alternate histories in PoW by agreeing to run a modified version of the client. There is negligible thermodynamic cost to rewriting history when you control a majority of the nodes.

Pow is really an incentive system to keep selfish miners flocking to one chain, by making betting on one chain more profitable than betting on lots of chains at the same time.

The PoS challenge is to do the same without the egregious amounts of wasted energy, by making miners bet their money directly, instead of indirectly through their computers and electricity.