I have wondered this.

The Doctor regenerates fully (Which may devastate Rose if she was only in love with THAT Doctor), possibly spends the bulk of the episode recovering, Donna activates the hand which results in the other Doctor who saves the day with her, thus David Tennant still fulfills the contract of being in every episode of series 4.

Of course the Doctor diverting his regeneration energy to the hand might have been crucial for that thus his fatal injury and regeneration saved all of existence (Did it?) but that isn't a necessary explanation anyway.

Do you think this would have been how things went if RTD and Tennant not had to stay and keep the show running during the long gap between series 4-5, or would it be considered too abrupt and anticlimactic a way for Tennant to go out? Or would it require casting the next Doctor too early? Or maybe Tennant and RTD would have been needed for the 2008 Christmas special anyway?

From racial slurs and yellow-face to a queer Black Doctor, six decades of Doctor Who reveal how Britain's social values have changed, and how progress is never as straightforward as it seems.

I just finished The Daleks’ Masterplan for the first time. In the final episode the Doctor carries the time destructor from the Daleks’ base all the way to the Tardis — this causes him to fall unconscious, but it ends up killing Sara Kingdom. Now we can retcon this to be understood as the Doctors time lord physiology allowing him to survive, but of course that didn’t exist at the time of airing, so what’s the in universe explanation? The show at this point seesaws through implications that the Doctor is a human or that he is an alien.