If the descentralization of social networks continue, we will have to prepare for the eventual rise of the instances wars, where people will start to fight about which instance is better and which one is weird to be in and so on, but that’s for the future of us all.
Well non-federated forums can grow by word of mouth and similar. Being federated does lower the barrier of entry for interacting but it’s still possible to visit the instance the old fashioned way. You probably still need to rely mostly on word of mouth anyway, even if you are federated.