Indeed, forums are almost gone. In particular, I miss one forum about science fiction, one about aeromodelism, one about electric vehicles (another still exists) and one about anarchism. An interesting hold-out in the country where I live, is a military forum, where rules say that respectful discussion is the only kind of discussion accepted - ironically, the military forum has a peaceful atmosphere. But it could come crashing down much easier than a social media company.
As for why forums disappeared - I think that people became too convenient. They wanted zero expense (hosting a forum incurs some expenses and needs a bit of time and attention), and wanted all their discussion in one place. Advertisers wanted a place where masses could be manipulated. Social media companies wanted people to interact more (read: pick more heated arguments) and see more ads - and built their environments accordingly. Not for the public good.
I think the most urgent job is getting rid of algorithmically steered social media - sites where one can’t know why something appears on one’s feed.
Within the I2P anonymous mix network there was an attempt, at some point around 2015, to build a system named Syndie where everyone would have to be their own censor, and servers would host content without the server operator really knowing or caring about what they host.
It failed to take off, but I’m not convinced if the reason was architecture or the main developer leaving.