> Email is being replaced by Slack

I don't know about this one. For example, I'm not going to invite every one of our clients to our Slack as a single-channel guest, nor am I going to be joining their Slack workspaces (if any of them even have one - most of them are in the Microsoft ecosystem). I'm also not going to do that with every vendor I'm in contact with, and so on — email is much more convenient. That being said, most of our internal communication within the company is on Slack, but that's a small slice of what emails are used for.