I would like to coin a new phrase, the "Game Master Dilemma". For any self selecting group the number of people willing to do thankless hard work decreases with respect to the difficulty of the task considerate with the reward.
In tabletop games there's often far fewer players willing to take the Game Master role because it's more difficult. The people that take the role have to really enjoy the task and the "reward" (good feelings because people had fun).
This means there's far more players than Game Masters. I think this applies to everyone making (in my opinion) pithy statements about "centralized platforms". There's vastly more centralized-social-media-platform users than people with the technical capability, time, and money to run some smaller social media instance. The ratio of players to Game Masters is huge. It can be a lot of work/expense to run a board for even a small group of users.
This means that larger more centralized platforms will end up being the norm because they centralize the infrastructure and lower the friction for users to do the interesting stuff like discuss topics or share cat pictures. Because the central platforms are being Game Masters players can flock to the platforms and network effects will draw more users.
If you want some magical world of super decentralized community moderated social media (you basically want mailing lists) you need to solve the Game Master Dilemma. Infrastructure is not free in time or money. The more demands on the infrastructure the more it costs. The only decentralized community moderated platforms that will exist are ones where the Game Master to player ratio is low. The higher it gets the worse of a job it is being the Game Master until it's not worthwhile at all.