But whether all miners get together and carry out a 51% attack or all validators get together for a 51% attack is basically the same, it is possible with both
Although on the surface it looks the same, it's actually not. Proof Of Stake is consensus where a Cabal of wealthy holders of the coin are in charge, a consensus inside the system. POW is consensus that's unrestrained by the system.
From what I understand, POS is no different from traditional consensus made up of Oligarchs. They HODL most of the system's wealth, then therefore control the system.