It *was* Overstreet. While I was still editor of the Guide, we did a year-and-a-half long project of communicating with collectors and historians to encourage new discussion of the Overstreet Age definitions as they stood and redefine them with some new additions. Since that time (it's been years now), there's a Copper Age in Overstreet. But we didn't invent it - hundreds of people participated in the discussion online, in traditional letters and in person, and the Copper Age had long been a term thrown around for the '80s well before us.
The Overstreet Age system is just one way to look at comic book history. You can do whatever you like, and if decades works for you, then go for it.