The long term influence of objects crossing or abandoned in GEO was evaluated, based on current practices, using the new Space Debris Mitigation model SDM 4.0. It was the latest version of a long term evolution code originally developed in the early 1990's, specifically designed to model with high accuracy the high Earth orbit regimes.Starting from the MASTER-2005 debris population, the long term evolution of the GEO environment was simulated under a few realistic scenarios, based on increasingly aggressive mitigation strategies. Beyond the high eccentricity objects, also the influence of the rocket stages separated from GEO spacecraft and left in geosynchronous drift orbits was analyzed. The main mitigation measures considered included the re-orbiting of GEO spacecraft at the end-of-life in the super-GEO graveyard zone, the de-orbiting of GTO/HEO upper stages and the re-orbiting of the rocket stages usually abandoned in drift orbits.These simulations allowed the quantitative estimation of the collision probability related to all the above mentioned classes of objects. The effectiveness of the current mitigation measures was assessed and the rationale of new mitigation guidelines devised to minimize the future collision risk in GEO was discussed.