I'm not a Discord user, but by my understanding it's ephemeral? Isn't that a problem for conversations people might want to refer back to years later?
If you had posted this on discord, and I'd been around, I would have replied immediately. If I hadn't been around, I wouldn't have replied at all. Given that it's a forum post, I saw it, didn't feel like putting in the effort at the time, and here I am replying to it the next day.
So to me it seems like, in terms of the flow of conversation, a post on discord is more ephemeral. In terms of storage, they're just as stored and awkward to search.
Like many forum sites, this one started as a front page of articles you could reply to with comments. Forums are a better version of that and took over. The current front page would be well served by generating a forum discussion thread for each article, say in the articles forum. Anyway the forums are a good version of article replies, but discord works fine for that too.
Discord is better for chatting, which turns out to be a lot of what people want to do - less-thought-out conversations that occur in real-time. Forum games, it depends, discord or forums could be better. If you want to re-read pages of mafia discussions, okay, forums, but I have to think people are playing mafia on discord right now and loving it. Things like tournament reports are currently handled by youtube, with some discussion happening on discord. Discussing strategy works fine on discord.
Discord has, by convention, a lower barrier to entry on posting. For some people it's the same - hey it's just a convention. But for many, a post is more intimidating, suggests that more thought be put into it. Now maybe you want to be reading stuff that more thought went into. I personally think the lower barrier to entry is way better. It encourages posting, meaning it's more likely that there's actually something to read.
So, I recommend joining the discord. I don't think discord is just killing forums - note that a zillion sites still have articles with comment sections. But for sure a lot of our traffic has shifted there.