@Sim would you have the courage to create a forum from scratch today?
Only if I had a niche I was passionate about, already had a lot of expert-level knowledge about myself (so I can foster the initial discussion while the community grows) and I thought was under-served and suited the forum platform well.
One critical differentiator between forums and social media is the ability to foster longer-form content on forums that just doesn't work as well on social media.
Social media is typically: short or long post followed by a bunch of short comments as replies. On forums it's not unusual to see replies that are as long if not longer than the original post.
If the content for your niche typically follows that pattern of short comments in response - then you'll have a difficult time competing against the incumbent social media platforms which already offer that pattern.
I did try and start a new forum a couple of years back, but discarded the idea after a year or so of operating the site, when I concluded that I was not able to offer anything significantly differentiated from what was already on social media. I may have persisted - but I also found that discussions in the niche were often very parochial and arguments would break out in the comments section frequently. In the end I decided that I didn't want to be spending all of my time moderating the fights that were based on ideologies rather than common wisdom or anything constructive. It would be like trying to start a forum on politics - if you're happy for people to just yell at each other online - then go for it, but otherwise it's just far too much work to moderate.
At the end of the day - I'm already working full time running my sites - I don't really have time to devote to a new site which may not bring in any meaningful revenue for a very long time and I certainly don't have time for any more hobbies.
If I found an under-served niche that had really good revenue potential and I was confident I could grow a community quickly, then I might take it on.