I had a hypothesis that within a given generation a series should not exceed three entries. The first entry is the introduction, the second the development and the third pushes the system to its max. Beyond that it is hard to get a leap from any successive entries.
The issue you have is companies are under increasing pressure to have reliable revenue streams. This is a big part of why annualized releases came about. Activision needed new Tony Hawk and Guitar Hero to meet short term gains, and aided in ending once beloved series. Call of Duty has the advantage of being a colossus so even if it declines 60% in sales over next few years I could see it still being very profitable. If I had to pick Activision's next victim it would be Skylanders and look forward to seeing bins full of those plastic figures for dirt cheap by 2016/2017.
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