The so called console cycle sees new generations of machines released approximately every 5 to 6 years, but having a production life of around 10 years. So the generations overlap. This enables the platform holder to have a two model range at different price points, an older, less expensive, model and a newer, more expensive, model.
Sony launched the original Playstation in December 1994 and followed it with the Playstation 2 in March 2000, yet they kept the original Playstation in production till March 2006. So for 6 years they were selling both machines. A few months after stopping production of the original Playstation they launched the Playstation 3, in November 2006, and they were back to having a two model range. The Playstation 2 is still selling massively well worldwide, and so it should, it has about 4 years of production left to run.