On the contrary, dear d5000, your definition of a forum is too broad. Are you comparing TikTok where people show their faces and do little dances with this anonymous forum? Or Instagram which is a social network where people show idyllic lives? On Instagram or TikTok there are hardly any discussions.
Fair enough 
I was more or less focusing on the technical side, and both forums and social networks and video-sharing platforms are simply "aggregators of public HTML messages". There are some differences of course, and as you point out, there may be different kinds of usage. But at least X (Twitter), Reddit and Facebook, for me, are forums.The main difference between forums and social networks is anonymity, and that the forum is written-only.
Anonymity is a feature of many forums but not all. I don't remember concrete examples now, but I have seen forums (more in the 00s and 10s) that had implemented a "real name" policy, even if otherwise they looked like your standard phpBB instance.And even the "written-only" requirement can be challenged. The user who has the highest post count on Bitcointalk is ChartBuddy, a bot who only posts images
Anyway, I don't want to insist on that. The point here was to discuss the possibility to create a decentralized version of Bitcointalk if theymos wasn't able to continue to run the forum. And I think in this case, a forum with anonymous accounts would make most sense, because of the privacy-friendly nature of the Bitcoin community. But I could see many features from Steemit, but also from X or Facebook enrich this hypothetical post-Bitcointalk forum to make it even more comfortble to use. Steemit was also anonymous and mostly focused on the "written word".
I also think such a hypothetical post-bitcointalk decentralized forum would have to find a way to preserve at least some parts of Bitcointalk, particularly the era before 2014, for the reasons mentioned by @JollyGood.