Yea, I can't see Matt Smith walking away before the 50th Anniversary, and I can't see BBC not doing everything in their power to keep him until at least the 50th Anniversary.
This close to the 50th Anniversary, I think it would be a mistake to have a new Doctor prior to it. New Doctors always go through a period where the audience needs to let go of the previous Doctor and accept the new one, and I think it would hurt the impact of the Anniversary to burden it with that adjustment period? Of course, they could always make The Anniversary be the end of the line for Matt Smith and bring in the next Doctor at the end of it
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?