Graham predicted Slack (email replacement) would be big before Slack existed (launched Aug 2013 https://en.wikipedia.org/wiki/Slack_(software)#History)
After our acquisition, we transitioned from small startup where it was super useful to a large enterprise (5500 employees) and I've found that Slack is pretty difficult to scale here beyond engineering/product management.
Slack is strong with helping small scrum teams collaborate, or fielding employee questions in town hall meetings. But, then there's #channel overload which compounds the distraction problem and feels just as bad as email with hundreds of unread messages. And, we have a lot of teams in completely different time zones (India), so the synchronous stream of communication doesn't work as well because it's difficult to track conversations. It's also not inclusive by nature; I have to invite external people to participate, which is a security concern at my company, so it's disabled and used for internal purposes only.
All that said, I love Slack but it is what it is. Asynchronous email is still very valuable to me, much more than Slack.
It was really important for us, because important decisions were being agreed in the midst of a Skype chat, which was only in your history on one machine and not easily searchable.
We also had Google Hangout chats/conversations which are searchable. However this caused further confusion about having to search via email/Skype/Hangouts to find out what was said. Also the conversation from one Hangout was separated from the other - where as we could replace that with one slack channel from all the Hangouts that we had.
Skype for Business now does something similar in that it also keeps you chat history in Outlook.
However I really like the clean separation of emails vs Slack chats. Slack is completely independent of my email so I'm not dependent on Gmail or Outlook.
For me it means that email becomes reduced but I regard it as a channel that is reserved for more important communication, i.e. I'll email something if I want it to be on record as having been written. All the chat fluff gets dumped in Slack.
IRC also has the added benefit of being scalable and free. And everyone has their favorite client already that is more customizable than any Slack client. And you can own your own data/conversations, as an organization. I have never really been comfortable with the notion that Slack can poke around in any channel's history regardless of how secret those conversations are thought to be.