Did you see people mourning the demise of forum software, when neatly maintained places oriented towards specific topics gave way to noisy and all-encompassing places like FB at Twitter?
I think these fragmented Discords are the return to the idea of specific, uncrowded, neatly maintained places, with a relatively high barrier to entry for a random person. Subreddits are a bit similar, but less insular.
Discord is still not the same and in my opinion inferior. It’s mostly synchronous chat with poor searchability, something very different from what forums used to be
One of the only differences between new Reddit and Discord is that Reddit has the courtesy of a public index.
I don't know much about Discord (my only experience being some years ago when I joined for an open source project and left soon after I noticed how incredibly use hostile it is) but I do know that if you create a single account it is trivial to join any "server" (which, despite the marketing is just a chatroom hosted on their servers).