This is really insightful. I know it's different when you're talking about private annotations, but in the context of the original dream of public annotations

This is really insightful. I know it's different when you're talking about private annotations, but in the context of the original dream of public annotations where it was hoped that you would benefit from benevolent and knowledgeable strangers who previously annotated the factual errors in the article you happen to be reading, I think you've absolutely identified the key problem. This is causing me to realize that the value of Wikipedia is not the fact that the pages are editable by the public; it's that there exists a process (as flawed and controversial as it is) for debating over the various contributions and edits that leads to a single, unified version of any given page. More than anything else, that consolidation process is Wikipedia. A successful consolidation process has to be present in order for any system of public contributions (annotations or otherwise) to produce a coherent result.