Now correct me if I'm wrong, but there is no official word yet from Mazda if there will be a new MS3. They have only announced their normal 3 4-door and 5-door. The only rumors we have of the new MS3 comes from magazine spy photos, and that's completely untrustworthy.
It seems quite a big decision to make on the '10 MS3 that is not officially backed by mazda. The spy photos could have easily have been a test mule for some other platform. Mazda does that all the time.
Plus, keeping with the rotating mazdaspeed model every couple of years, it's time for a change.