I'd say it's more of a failure of certain search engines, among other things.
Personally I've found reddit karma to be correlated with the obnoxiousness of the
user, not the inverse. There's plenty out there who feel the same. Even with that point aside, what is there to prove? I don't understand the assertion. Wrongthink will get you censored on Reddit faster than anywhere else regardless of how much karma you had.
Even good posts on Reddit get shuffled past the first pages after a few days. There's too much negative incentive to put effort into content when it can get censored, downvoted, or buried. Sure, you might potentially reach more users in the right circumstances, but many of those users are addicted to notifications and doomscrolling, not enthusiasts of the subject matter. Plenty of that here on HN as well (more on the side of trend-chasing, manufactured two-minute hate about climate or national news, while rarely challenging the status quo) but it's at least tolerable, and I know the moderation is fair here.
Forums haven't "lost out" when they can still serve to bring people together who have, or want to read unpopular opinions. I could go on about use cases. If there were none, there would be no forums.
The takeaway here is that Reddit can be and is considered by a decent proportion to be a dystopian failure, not a "convenience". A black hole of siphoned search traffic that regurgitates a watered down and sanitized imitation of something resembling a community.