It’s why we re not using smtp for chat. SMTP can’t be extended enough so replacements are built instead. Similarly if signal federated, eventually it would freeze and a few years later users would move to wherever they could get new features.
Federation is a good thing but only when the protocol is finished or if there is a forcing mechanism to allow updates to the protocol. ethereum/Bitcoin are good examples as they have flag days that force the value of currency to be in the balance to keep the protocol moving forward.