I like our chat system (AKA Bonfire).
It's simple to use and not too frilly and complicated. I've used it more than I've used Slack and Discord but I've used both and I'm not a huge fan of either. There are some things they do better but, in particular, Slack's odd threading just means you miss content from time to time.
As far as I'm aware, there's no burning urge to somehow swap in either of those for our in-house chat system... and I'm not even sure how we would do that in the first place, so... no.
It's also worth keeping in mind that both of those other products are built to allow private discussion in a segmented, private chat area. Our chat system is default public. While Slack and Discord may have public chats, that's not a part of them I've ever used. From my experience, the content on them is default private.
In contrast, we explicitly prevent the creation of private chat spaces in Bonfire - or at least restrict such to our moderators, entrusting them to use them for moderation purposes. We do not allow one-to-one private messaging in particular as it would require far more moderation than we currently have.
There are other problems with it, particularly privacy protection. Many users here do not want their accounts connected to their person in any way. That becomes more difficult as you add connections to external products that users may utilize for work, particularly Slack.
Some of our sites do have unofficial private chat spaces off site as places for the community to interact but they're non-sanctioned and we do not support them at all. We also generally discourage sending users from our chat to those other services because they're unofficial and unregulated (by us) and that's often unclear to the people being linked to them.
That said, for our non-public content, we do have a Slack integration for posting questions from a Team into a Slack Channel to help our users get visibility for their internal questions but this is a far cry from actually bolting Slack onto Stack Exchange.
