This is a moot question, because the only thing that matters is engagement.
Internet forums:
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Are (mostly) specialized.
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Don't have S&P 500-level advertising.
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Don't constantly ping your phone by default.
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Don't incorporate "dark patterns" in the UI.
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Don't have AI-based algorithmic feeds.
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Have rules for posting, which slows down posting.
People didn't give up on them; their attention got sucked away to other things. And nothing could have changed that, no matter how much they improved.
Not that I have any high horse to stand on.
For a few years, I've been telling myself I will become active in a few still-extant forums again (like DPReview and SpaceBattles). I never did.