As many have said, the utility of a tool like slack justifies it’s continued use in our community. But, I’m very much in favor of this conversation and look forward to what we all can learn from it.
There are many good points already mentioned above. I’d like to pull a thread that hasn’t been explicitly addressed. My thoughts revolve around two audiences: 1. new ember developers and 2. the larger javascript community.
The Learning team has done a fantastic job improving the guides to help guide new developers 
. However, there is no way they can answer every question, nor should the guides try to accomplish that. Having Slack as the primary channel for questions is not only intimidating, it’s inefficient. The signal to noise ratio is astronomical. This adds a lot of friction, especially in those critical early days of learning of the framework.
Another downside of our slack usage is that we’ve effectively walled ourselves off from the larger javascript community. I regularly have to respond to folks who say “I thought Ember was dead?” I believe that is a consequence of how much of our relevant, collective knowledge is not public in any meaningful way. This forum kind of feels like a ghost town. So many of the answers on stack overflow are from pre-ember-cli days.
The article @ef4 shared above about how Discourse uses Discourse, is instructive. Slack is ephemeral. The core team and addon authors should push Q&A to here or StackOverflow. It should not take long for the community to shift gears.
(Side note: I didn’t realize how capable Discourse is. Check out these two sections as potential sources of inspiration for here: howto - Discourse Meta & plugin - Discourse Meta)
I hope these thoughts are helpful. Go team! 