I wanted to weigh in here with my 2c, for what it’s worth.
Firstly I don’t like how this conversation is ending up as “should we ditch Slack for discourse”. I know some people are giving measured responses but my overall takeaway from this conversation is that Slack is broken and we should throw it away. Why can’t we just use both!?
I agree that there are some conversations that would be better served on discourse, and we have been experimenting in the Learning Team channel by every-so-often bouncing overall discussions out into discourse threads (like this one for example). Remember, as developers, we know when to use the right tool for the right job and we should try to apply a little bit of that with our communication system.
Another thing to remember is that there are going to be some cases where people appreciate the more direct touch of using something like Slack. Asking a question and having @locks reply to you could be the nudge that makes you want to stay an Ember developer 
The flip side is also true, I know that having a slack channel with 1K people in it can be intimidating.