The lower level is centralisation into a small-ish pool of large services where no service exceeds 51%. That's what I was mainly thinking about.
We've seen in the past how Bitcoin has adjusted to the threat of 51% with people withdrawing capacity from large pools etc. in response.
In either case the point is that the existence and open availability of the technology acts as a deterrence to coercion because the act of trying to take advantage of a 51% attack will send people running for the hills (and the existence of altcoins makes that even easier).
We've seen people willingly pull back from potentially even "accidentally" exceeding 51% of the Bitcoin network in the past for that very reason.
The point is not absolute decentralisation at all cost (though some anarchist tendencies do want to maximise decentralisation), but the ability to withdraw consent and unilaterally decentralise.
I'm probably misunderstanding something, but isn't it more that blockchain technology allows decentralisation, while dominance of any particular deployment supports centralisation? As I understand it, once Bitcoin is centralised, the 51% can control it as they wish, and the only alternative is to set up a wholly separate Bitcoin2 chain, or another altcoin.
(Again resulting in cryptocurrency being a bad store of value.)