Maybe the non-engineers don't have much of a reference on what a better solution would look like.
I also use both as I'm a partner on the engineering side of the business but also run a large consulting pod - I really don't care which one anyone uses. I don't personally see Slack being better 'enough' to justify the effort of switching for those that are already on teams.
It would be a dark day indeed if we'd have to stop spamming memes while at work