With the number of GitHub discussions where I’ve seen the OP or the maintainer screw up the threading, as if it’s a normal issue, I’m increasingly convinced that it’s just a bad idea to have both threading models under the same roof, whatever the merits. A separate site would require more overhead, but if we can manage it, then I think it would be a better experience overall.
This is probably also a cautionary tale for if we ever enable Discourse Chat here. That plugin offers a choice of simple replies (like Discord) or full-fledged threads (like Slack). We would probably want to stick to one or the other rather than having the rules change from channel to channel.