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.
The big problem is going to be when someone decides to start spamming and vote manipulating with bot populated private instances that automatically re-spawn themselves under a new name whenever they are blacklisted. Eventually, the standard will have to move to whitelisting over blacklisting, and once that happens the whole premise of federation starts to fall apart.