The main problem with centralisation

If the majority of user and communities live on a the same instance, and that instance goes down or becomes to toxic to keep federated then fediverse loses a large chunk of its whole. While if it a tiny instances that dissapears only a few people and communities need to migrate.

General i also think its beneficial if the owners of an instance are the same people from the communities its hosts. The role of general use instances should be more like a backup mirror

One example is the lemmy.ml problem. You see the rest of the Tankie Triad is pretty widely defederated from, but lemmy.ml isn’t despite doing much of the same things, just a bit more subtlety.

Because lemmy.ml is such a big instance and has a few of the biggest comms (By subscribers only, which means a bunch of dead accounts boosting the numbers, by MAUs a much different story unfolds) many instances admins are hesitant to defederate from them

Big instances carry too much power and influence and if you’re a toxic instance like .ml, it can be used to basically bully other instances into remaining federated with you