I don't think you can assume everyone wants to be on discord. I certainly loathe adding yet another discord or slack community that frankly I don't check. Nobody has time to keep up with dozens or hundreds of discord communities (it's very easy to join one).
I prefer any online community that is searchable (via Google, site search, etc) so that I can find answers and past discussion without having to ask the same question for the 100th time in the channel.
How is Discord more social than the other systems you mentioned? I consider something to be social (social media, social network, etc.) when the primary utility manifests as a function of establishing friends, followers, or whatever similar jargon.
That is: if the content presented to me is primarily generated by users who I've selected, while content generated by users I haven't selected is unavailable or relegated to lower tiers of functionality, then it's a social network/medium. In other words, it's the product of subscribing primarily to people (regardless of what they might discuss) rather than to topics (regardless of who participates).
I don't see Discord in this way. Isn't it more about subscribing to topics than to people?
I realize you're not speaking for yourself, but for the HN hivemind; my question remains.
Discord is bad because if I don't like the default client, I can't use or build another one. I can do this with IRC/XMPP/Matrix.
My personal fear with Discord is the audience. Discord has a lot of kids and the likelihood of having kids come into your server and troll you or ask low-effort questions is much higher than Slack. But if your users want Discord, then you should use Discord. There's nothing gained by telling your users what to like.