I don't think it's unsolvably flawed. It's just hard as hell. If you take a structured data approach to annotation and commenting, you can use analytics to weed the naturally-occurring power law distribution of comments down to just the best ideas on any particular sentence, for example.
Then you fail to activate one of the most important aspects of community building when people post comments, which is people feeling like they're participating and making a difference because other people respond to them. Take that engagement away and you're going to have a hard time building a user base.
The problem isn't that any given aspect of the annotation problem is unsolvable. The problem is that you can't solve them all at once to build a successful system.
Recall that I said I've been "watching this space" for 18 years now. That's no exaggeration; I was the first to hack Third Voice back in the day. It's not like people haven't been trying. Speculating about how it might work someday is really kind 10 or so years too late. In 1999, theorizing about how cool annotations might be was adequate. In 2017, a theory about how cool annotations might be needs to not just explain how they might be cool, but why all previous efforts have failed, and why yours might succeed. That's a much higher bar to leap now.
And one obvious solution is to trim away bits of the problem, but you end up in the local maximum of "link aggregation site with comments" pretty darned fast.
