In addition: From a users perspective having one account for reddit which replaces x accounts for specialized forums makes things easier such as managing notifications, etc.
That forum I signed up for in 2012? I don’t remember the name.
That Facebook group I joined in 2012? I still see the occasional post in the feed.
Also it’s vastly easier to find these communities in the first place. There’s no way to look for forums about a specific topic, if that exists at all.
Forum platforms could offer the same feature by interoperating with the Fediverse. You can sign up on one server and use that one identity to post content elsewhere. (Plus it also addresses the issue of needing a mobile native UX, there's plenty of apps that can interact with the Fediverse.)