Using the words "centralized" or "decentralized" when describing crypto (and probably many other tech things) is perhaps frequently the worst oversimplifications one can engage in.

I think what you want to discuss when using these words is the extent to which it is either likely or possible that a small number of players can unduly influence a thing, but there are frequently MANY MORE FACTORS at work here than "that number." You have to look at how the thing is set up, what governance looks like, what technical limitations there are, and -- perhaps most importantly -- what incentives are there in place for the players to do so.

A lot of the "centralization" fears are bad because they're the sort of thing, that -- if executed -- would destroy the value of the thing to the centralizers themselves, and thus they would literally never do it.