The major problems I see with slack and similar is that it is too easy to lose messages. If I click a channel, but get distracted before I can properly read or act on it, the message is effectively gone forever (unless I remember, or the next time I happen to open the channel). No other medium has the explicit "delete" (or archive or whatever) that email has. I find this way way way more useful than the "read" concept. Because read means nothing to me, even if I actually did read it (not that it was just present on my screen for a couple of seconds) it is incredibly likely that I forget about it.

Other things I like about email: - Multiple clients let me use email how it works for me. I can control how it notifies me, how it sorts my messages and everything else I care about. Slack is ok, but I don't really believe that one size fits all. And I'm not sure that it even fits many people. - I can send an email to anyone. I don't need to invite them to my workspace or channel or anything. Just enter an address and it is off.