I'm surprised no one has tried to revive the Wave idea in some way. Whether by forking the original project, or attempting a remake.
Slack is also disruptive in a way that email isn't. Slack demands attention, which breaks concentration on other tasks. Email can be dealt with at the recipient's leisure.
I'd really love to see something similar to Wave's "real-time" instant messaging in one of these newer apps.
i.e. the stuff a veteran IRC user cobbles together from an always-on server running irssi+screen, nickserv, saved identify and ghost commands, a chatbot and some personal webspace.
Still, I agree Slack is far more the child of IRC than the child of Wave. IIRC Wave had full rich text, a threading-like mechanic, and you could edit other people's messages, and documents rather than channels.