Wikipedia is open source content though, which was a big aspect of it being successful at the beginning (as in, many of us wouldn't have bothered to write content for it had it become proprietary).

Open-licensed content cannot be said to be controlled by one party, as anyone is legally entitled to create a fork. Thus, Wikipedia is not controlled by the people who run the servers; the infrastructure may be centralized, but the project (the compilation of knowledge, and the articles produced) is not. This aspect of control is what I said you were missing in your assessment, and it seems I was right.