Earlier quoted context omitted.
Slack would become the best chat app for every company out there with just one simple feature (or the lack of it): - remove the "user is online" green dot. Really, it's not needed. Once it's removed then everything becomes async by default (and so everything slows down a little bit =>everyone is happier)
E-mail is inherently async. Async to the core. Why not just send your async requests as an e-mail so the other person can handle it as they process their inbox? Slack excels at real-time, interactive communication. It's chat, designed for back-and-forth. Your messages will get scrolled into the backlog if there's a lot of chat while you're away. The green dot (or the "busy" setting) is great for understanding whether…
Email doesn't have an easy way to add someone to a thread, or to manage a forked conversation. It's closed and non-discoverable by default.