Slack doesn't replace email. That's a silly marketing slogan. Imagine for a moment if it really did. What if Slack became so popular that everyone stopped using email? That would be horrifying. You wouldn't be able to communicate with anyone without using Slack because it doesn't federate with any other service. Every website would have to integrate with Slack for user account registrations. Etc. Decentralized federated communications is the backbone of the internet. The fact that IM services have never broadly adopted federation dramatically cripples the potential of the medium with horrible fragmentation and balkanization.