It's like making it mandatory for restaurants to have a live updating Yelp! review display scrolling at the front door.
(assuming I understand the proposal correctly...I didn't see much control there for website owners)
I personally think if that is done it should require the user to purposely choose an annotation service as well. The actual act of inviting the conversation to pages should involve a user choice.
See further thoughts on the question of page owner consent-to-be-annotated in another of my replies above.
It's more like AR technology existing and allowing consumers to have any review source they choose accessible and popping up live reviews as they walk by businesses (actually, without the actual AR part, mobile virtual assistance like Google Now provide that today based on geolocation, so much it's really something that already exists in physical space extending into cyberspace); but, yes, you control your content, but not what other people say about your content, and not how other people find and share what other people have said about your content.
I'm not saying it's necessarily bad.
It is, though, more discoverable than anything preceding it in this space, assuming browsers leverage it. It would likely create a fair amount of churn.