We don't need to discuss that masters of Bitcoin are two-three biggest mining pools, do we? While in NXT over 300 individual accounts successfully generate blocks. Well, who is more centralized?
We don't, because it doesn't matter.
In a PoW system, if the three largest mining miners decide to misbehave it's at least possible in principle for somebody else to build out more hashing power and compete with them. The miners can not block competition via any other method than expending more real opportunity cost to do so.
In a PoS system, if the majority of the stakeholders decide to misbehave then the system is fucked. Stakeholders are insulated from competition because they only lose stake if they choose to sell their coins to their competitors.
In a PoW system, if a cartel forms it's an annoyance.
PoS systems are inherently designed to form and protect cartels.