When Slack is bought by Google and merged with Hangouts, and WhatsApp changes over to a new all-messages-delivered-by-the-NSA protocol, email will still exist.
I've recently been thinking about xmpp making for a great federated identity provider for multiplayer video games. With communication already built in, initialization of peer-to-peer sessions already a common use-case, and the right XEPs it could replace centralized services like steamworks, psn or xbox-live.
And XMPP has always tried to be the email of IM. Similar model, address format, and so on. A sea of independent, autonomous domains openly connecting to each other.
