I think that Slack would argue that they are disrupting email. We have many fewer internal emails at work now - most are directed at clients.

Facebook Messenger, WhatsApp, etc. Are strong contenders for replacing email to friends.

Not trying to avoid your question, but I think the future of email isn't a better protocol or SMTP client enhancement, it's apps that provide a better way of communicating a given task.

As it turns out, the major feature of email (being able to message anyone) is also a huge drawback (communications inundated with spam).