I guess that's a "good" alternative if your goal is to prevent the evolution of deep discussion and instead get people to move on and scroll over more ads.
the reason we do the "one big omnithread" for everything nowadays is because we have moved to a fundamentally reactive model, where youtube opinion leaders or news articles say a thing and people react to it, most people are the metaphorical AIs who don't have any independent thoughts until prompted by an external stimulus.
Actually in practice any internally-generated questions tend to be smashed down on reddit. Self-posts end up downvoted heavily. Some of that is that they're often low-quality content asking dumb shit that should have been a comment on some other related post rather than a full thread, so it's the "stupid newbie" downvote. But they also don't have anything to consume, and again, the 90-9-1 model applies. 90% of your users are readers, 10% are commenters. And actually beyond being a reader, they usually are not reading the comments either, they're reading the articles. While in fact the commenters often are the exact opposite, they don't read the article/watch the video because they're there to talk.
But the fact that 90% are not there to talk, and actually they don't even care about the comments at all - that's why Reddit is pivoting. Forums discussion is a smaller niche than content drips.
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when you do a single megathread and content is moving super fast within a single thread (eg - politics debate-night threads), you have to resign yourself to not seeing every single comment in a thread, because it's literally going faster than you can read. And that's also true of reddit - you're not seeing every comments, you're seeing the top 50 comments. And if you sort by "new" it's a similarly torrential feed, and you stand no real chance of reading everything let alone replying to everything - but the "top" and "hot" and "best" sorts do a good job of surfacing the content that readers want to see, and it feels stable, so it cures that FOMO.
Technically there's no reason that's not a feature you couldn't build into a forum though. Surface me a view of a thread that's only comments above 10 karma, or with more than a certain number of replies, and show me a "top threads" view of a given forum. And reply notifications are not a novel feature.
but anyway, your question gets at another tangential point, which is that reddit and reddit-style models have become so dominant over the last 15 years that people literally don't know the ways to interact efficiently on web-1.0 forums anymore. People know how to work reddit, they know how to work twitter, they know how to work discord... does the average user know how to work phpBB anymore? probably not really!
