Way back in the day, Reddit was marketed as a way to create a forum without any technical setup, and then to view content across all the communities you were a “member” of in one place. You can still see glimpses of that, I think when you signup or register a subreddit it suggests making one for your dnd clan or something like that.
I don’t think Reddit realized how smart this strategy would end up becoming, because over time what happened is that existing Reddit users would just join the subreddit for some topic they were interested in (like a band) instead of seeking out forums for it on the web. Eventually enough people started doing that, that subreddits for a topic would absolutely dwarf any single traditional forum in activity. Because reddit is a single site they could also do a bunch of SEO optimizations due to having larger scale than independent forums. The result is a massive network effect and controllling the top-of-funnel for online discussions on the web.
Anyway, what fucked up Reddit was that after 2010 or so they started turning it into more of a consumption platform than a discussion platform, plus various issues with moderation (the emergence of powermods who are all secretly monetizing their subs to help advertisers, implementing platform-wide moderation policies), turning it more into a centralized service with less focus on discussion.
Honestly, combining the web with forums provides like 90% of what you want, it just doesn’t do aggregation, nor does it let you directly ban a mod (but you can always switch sites). But it does decentralize auth, moderation, servers, and allow more freedom in how a subreddit-equivalent gets run while also allowing moderators to directly monetize their sites rather than having to resort to secretive scheming. Maybe we simply need some kind of protocol or common API + FOSS client that lets you automatically grab and sort top posts from a list of forums that implement the protocol.