So David Tennant has announced he'll be leaving Doctor Who after next year's specials. (BBC News: David Tennant quits as Doctor Who) Well it had to happen at some point and by the time we reach the last one he will have been playing the role longer than seven of his predecessors (give or take but I'll leave the debate about how long Sylvester McCoy was in the role to other Who fans).
Doctor Who has always been about more than any individual actor and so has survived such changes before and I have no doubt it will do so again. The idea that the series is so fragile that one wrong move and it will be "Ruined FOREVER" is more laughable today than it has ever been. The series has come a long way, outlasted so much and even converted its greatest enemy. (See Michael Grade admits he was wrong) It has become a modern myth, reinventing itself for each successive generation, and I have no doubt that we will see many further adventures of the Doctor in years, decades and centuries to come.
As for David Tennant himself, he's had a good innings and some of his adventures have been astounding. But as Disraeli once said, it is better to leave people wondering why you did not go on for longer than leave wondering why you did.
Here's to the next Doctor, whoever he or she may be.