It's interesting to look back at the last decade and how things have changed. I think it's fair to say that discussion forums in general are mainly older boards that have survived to this day, rather than new ones that are sprouting up. Here it seems that the next generation is content with just setting up a social media group and cannot be bothered with developing their own site and community.
In terms of software I don't think there is that much of a difference in terms of the user experience between various forum platforms from a typical, old school forum user's POV. It's only the young generation that needs everything linked to social media and the huge emphasis on making the experience mobile-friendly that sets software solutions apart.
Otherwise I think a simple design/layout update to make everything more sleek and up-to-date could easily do the job. It's more about refining the existing experience and stereamling current features rather than revolutionising anything...
There's still a good number of folks who have been making new boards. But I get what you mean though, sadly more people would rather open up a discord server or facebook group. The good advantage that anyone who owns their own forum has is that you're in control of it and as long as your hosting stays up and you don't shut down your forum, it'll remain online. Whereas the people who choose to use Discord or Facebook Groups are running on borrowed time so to speak. If their servers go down, goodbye communities! I wish more people would realize that. I do hope UBB threads can be integrated into the Fediverse via Activity Pub someday. That would be cool! So far there's only two forum software that have this integration, and they're both nested forums that look similar to Reddit. A threaded forum like UBB threads would be good to see in the Fediverse. (The fediverse is a decentralized network of social media platforms that are connected via Activity Pub.)