No argument. Have been writing about this a lot within the context of Bluesky's claims about being decentralized. There are two aspects to 'decentralization', one the technical structure of the network, and second the actual user distribution and commensurate power differentials. A decentralizable network where users are all consolidated on one node, is not decentralized at all. Or, as you point out, in the case of Mastodon (and email), only imperfectly decentralized.