Even if more than one major pool was owned by the same entity, it doesn't make a huge amount of difference. Remember of course that each big pool is used by thousands or even tens of thousands different miners - everything from huge ASIC farms down to individuals running a single ASIC at home. All of these miners are separate entities despite mining under the same umbrella of the pool they use. Should that pool operator decide to turn malicious and attempt to 51% attack the network, then any of these individual mining entities can switch to a new pool in a matter of minutes.