I disagree with you that Slack is an ergonomic interface. I'll give two examples off the top of my head:

  1. New Threads sort with new items at the top.
     This is opposite of everywhere else in the 
     app. And there is no indicator, e.g. the 
     date isn't displayed.

  2. Try backspacing over an @ mention or two.
     It is very unpredictable behavior.

I could find many more if I had Slack in front of me. I'm not saying that IRC is the solution, but I can't believe that Slack is the best we can do.