Proof of Work is not resilient to a sufficiently large 'cartel' either in this case. If your cartel is 51% of bitcoin miners by hashrate, they can agree to let each other spend their bitcoin twice. Of course, this is sufficiently hard to do with proof of work, as with proof of stake, as to make it infeasable, perhaps even moreso with proof of stake.
As an example with bitcoin specifically, it would probably be cheaper to buy enough mining hardware to constitute 51% of the bitcoin network than it would be to buy 51% of all bitcoin.
Of course, both would be very expensive endeavors.
