Googles stated mission is "to make the world's information universally useful and accessible." By definition, this would mean to filter out disinformation. Your interpretation of this as an "aggregate credibility index" is demonstrably antithetical to the mission of spreading information. If the entire planet placed credibility in "1+1=7", the google calculator should not be rewritten to reflect global credibility scores.
PageRank counted the number of inbound links to a page under the theory that the resources people were organically electing to point to were superior than others. This was and is the foundation of Google's search, and it's what allowed them to succeed against massive incumbents like Yahoo.
The concept that the general public can tell what's worthwhile is the fundamental component of Google's success (as well as the theory behind news aggregators like reddit and this very platform). The theory was that most people would intrinsically reject falsehoods like 1+1=7 and that they would intrinsically gravitate towards facts like 1+1=2. It was an investment in the wisdom of crowds, in democracy.
Perhaps this concept is no longer useful, but top-down editorialism is a departure from what most people perceive Google to be about, whether you agree that it's a good direction or not.
