Also, if you're relying on a stupid cat gif to judge your colleagues mental well-being (and you are concerned about their mental well-being), perhaps you need to spend more time talking to them and less time searching for dank memes.

The key thing Slack offers most 'customers' (even those who don't pay) compared to something like IRC is the simple setup/use.

I say this as someone who hates Slack, but has had to endure it on a number of client projects.