Slack’s revenue is firmly linked to enterprise, so I don’t see this as a strategic error. It is, however, a fact that open developer communities are increasingly using Discord.
I also expect more Matrix adoption in the next 1-2 years.
Although Discord open sourcing at least server instances while still following their federation model could be interesting. Or interop with the Matrix protocol.
I have a draft post that I’ve been meaning to finish for months of how Fission uses Discord chat + Discourse forum. Soon!