I disagree that having a popular flagship federated service negates decentralization.
The point of decentralization is not to destroy the ability to centralize (see e.g. git versus SVN and then look at github).
The advantage is that federation and decentralization empower an entirely new set of use cases, archival strategies, development, and accessibility affordances that may have not been possible before. While enabling the town square & metcalfe's law that are advantageous to many people / use cases.
The point of decentralization is not to destroy the ability to centralize (see e.g. git versus SVN and then look at github).
The advantage is that federation and decentralization empower an entirely new set of use cases, archival strategies, development, and accessibility affordances that may have not been possible before. While enabling the town square & metcalfe's law that are advantageous to many people / use cases.