Jonathan Blow

There's a high chance to not get the best possible answer because the person knowing best might not be active in that very minute and, at a later point in time, the question is just not visible anymore, wrong answers may not be corrected by someone at a later point in time.

Once an expert member leaves, the knowledge is often downgraded to less experienced users repeating what they remember with a high chance of losing/missing details over time (Chinese whispers).

Potential new users will have no way to get a feeling for how active/valuable the community is, maybe leaving immediately as it may seem kind of abandoned. It's next to impossible, to browse the history on Discord to learn something.

Compare this with f.i. a simple, reasonably structured forum + subforums. Browsing just a few pages of thread topics, reading the contents of a few interesting ones etc. is just so much more valuable.

I think we have very different standards of quality for what is worthwhile to archive long-term. I would not, from extensive experience, consider forum discussions to ever be useful as learning/reference material.

If you want something to be genuinely useful as reference or learning material, you improve its quality and put it in something people might actually bother reading, like a book, a talk, a wiki, or even just a high-quality blog post. Not a forum thread.

In practice, one of two things happen: either people on the forum take the archival usefulness of the forum seriously, in which case you stifle many kinds of useful discussion, like personalized advice, and candid non-Q&A-style discussion, things discord is particularly good for,

or people don't consider archival usefulness to be a priority, in which case you get all of the same problems you're worried about getting in a discord server, but shoehorned into a structure that does not match the reality of the conversations taking place.