One of the arguments against proof of stake is that those with the most stake maintain power over the system (much like our current financial system) and therefore is not "truly" decentralised.
I don't think that is a valid statement.
There are degrees of decentralization. It is not just true or false, so there is no such thing as "truly decentralized".
Power will never be equal across all participants in any system, decentralized or not. And while power distribution is a factor, it is not the only factor.