We’ve been using Slack a lot recently in Kite. A lot. I haven’t sent a company email in a year, probably.
However, after using it for so long, to me there’s a clear problem that Slack doesn’t solve and email does. Slack messages don’t persist. They’re brittle in nature and they rely on the spontaneity of the communication. It’s hard to go back to them or to relate to them hours or days after they were sent/received. (And search in Slack is not the solution, it’s just a patch). Slack messages are great for many things, but not for information that you want to persist (currently the only ways to make a message persist across time are “favorites” and “pinned”, and both are again just patches).
It seems to me that people still want communication that is less brittle, that resides in a clearly defined space, and where its nature is discrete and clearly encapsulated (as opposed to messages, which require to see before and after to understand the context).