The authenticity of old fashioned forums is often outweighed by their poor UX and in general terrible ergonomics. It's no wonder that so few people want to use them anymore. Reddit's "nested, collapsible comments sorted by upvotes" format is simply superior.
20 years after Reddit started, the best that the forums can offer is perhaps discourse.org, which is barely any better than traditional forums – sleeker UI for sure, but it's still fundamentally the same unworkable linear format. It's like sticking to magnetic tapes in the age of SSDs.
Even Facebook, one of the dumbest discussion platforms, has nested comments. Terribly implemented of course, but how does the platform designed for the lowest-common-denominator kind of user have more advanced discussion features than forums made for discussion connoisseurs? It is utterly baffling.
"Nested comments sorted by upvotes" is, for free and frank discussion, inherently far worse than non-nested in-line comments. With the latter there's no hive-mind effect, no consensus-seeking, no dopamine/approval-chasing. Also, traditional forums tended to encourage longer-form posts (which you can still see in places like Spacebattles), which naturally contained quite a lot of technical detail and pictures, whereas Reddit (and HN) are optimized for very short comments. In Reddit's case, smarmy one-liners, usually.
But the main problem, to repeat for emphasis, is that the upvote/downvote system (even if it's fair and used virtuously, and it usually isn't,) stifles disagreement and debate.
I strongly disagree. But maybe because of a difference of perspective. If you're imagining a Reddit-scale forum, with millions of people with no sense of community and no knowledge of the content they're consuming, then yeah a traditional forum format is awful.
Forums shine as spaces for focused communities, where people have reputations and care about the subject matter. Time-sorted discussions are great because that's what's happening - a discussion in the community. You don't want to read someone's quip first, you want to get the whole context. You don't want there to be upvotes that people try to earn - there's already your reputation in the community. If someone's a troll or gives bad advice or is wrong, they'll get called out, or banned, or simply ignored as everyone knows they aren't respected.
Forums just aren't meant for generic content and it's not because of the UI, it's because the entire concept is not compatible with masses of semi-anonymous users with no commonalities.