We use both Slack and Discourse, for very different purposes. It’s very unlikely that if one started picking up features from the other that we would use them in that way.
I go to great lengths with new staff members to emphasize that Slack is to be used for any real-time, ephemeral communication, the high-volume low value junk which will be irrelevant within a week. Beyond that it’s the home to our integrations, but again these are very much point-in-time, with no lasting value. For this reason it’s right that Slack has and relies on a local app, push notifications are what keeps people engaged.
Discourse, on the other hand, hosts all of our persistent discussion, knowledge, and article comments, and typically represents a lower volume of higher value stuff. We’ve also begun to use it for certain external communication. Partner organisations can register for accounts and participate in discussion directly on the site, or via email as they did before.
One big benefit of the above is the different pools of search, when I search Discourse I know I won’t get results from all of the cruft discussed day to day. Likewise when I search Slack, I’m not faced with the lengthier posts I’m typically not interested in.