I'm in the camp that Slack is a great UI for work chat, and that work chat is for low latency, low bandwidth communication.

If I want high latency, high bandwidth, there's email. If I need low latency, high bandwidth, that's an in person whiteboard session or a video call.

That does mean I think Slack is overhyped... but that also means I think Slack does something valuable. Just maybe not as valuable.