Another possibility is that once a player has completed all possible research and DOESN'T then attempt to hit Cydonia within a certain time frame (according to difficulty level?) then alien attacks upon Earth and Xcom bases will continually increase in frequency and difficulty. If you've completed the research tree then at most you need a bit of time to find and train up psi troops and to build ONE Avenger. Maybe 1 month, after which penalties for scoring purposes will increase, awards for any accomplishment will decrease, size of UFO's to intercept starts scaling up and you no longer even see Very Small or Small UFO scout missions, infiltration missions increase, etc. In short, you get 1 month grace period before the game starts sliding into INEVITABLE defeat short of cutting off the head at Cydonia.