Conquer the world! Integrate multiple systems. Federated systems should work pretty much integrated. Legacy systems users may migrate off, and try to keep some access by using their passwords. The only purpose of supporting this is the network effect. This should be done by using plugins or gateways. Users that don't want to get out of legacy systems won't be able to subscribe to federated users. If the legacy system is a silo, it won't be possible to subscrite to its users without an account. Pick up a single native protocol and write gateways.