I don't think the articles premise is that Google axed all content older then 5 years or so. But that it gradually discards old unique content.
Which goes against the original mission of Google to "organize the world's information and make it universally accessible".
A "bug" could be an option, but I don't expect that to be the reason. It's too easy to find examples of forgotten content. And I don't think a bug of that magnitude in Googles core business would go unnoticed.
Search. They still form a major part of their business. Through direct ad revenues but also to redirect traffic to other Google products (e.g. Maps, Youtube).
Which goes against the original mission of Google to "organize the world's information and make it universally accessible".
A "bug" could be an option, but I don't expect that to be the reason. It's too easy to find examples of forgotten content. And I don't think a bug of that magnitude in Googles core business would go unnoticed.