The power-law distribution of web pages wreaks havoc on almost any attempt to distribute people well here. Your solution basically would make it so the annotation works OK on only those pages that are on the high end of the local power law distribution, but that is probably fewer pages than you might think as you'd all have different long tails.
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.
Annotation in a weird way is even harder than a "boil the ocean" problem; you have to have the exact right number of participants, across all the enormous complexity of the Internet, or at least, a very huge chunk of it. If you have nobody on some page, the annotation system is useless, and if you have thousands of people pounding away at the same page, it's pretty useless too. You have to hit this window of reliably having just the right number of users on a page, on an Internet that intrinsically tends towards power law page viewing, somehow across the entire Internet. After ~18 years of watching the space, I simply believe the entire vision is unsolvably flawed, especially in the presence of successful link aggregation and commentary sites.