You mean aside from tradition and the fact that revealing a new Doctor for the BBC's hundredth anniversary is one hell of a promotion gimmick no one in their right mind would pass up.
But ignoring all that, because I know you are, there's also the fact that it's already been confirmed Whittaker has filmed her regeneration scene. Which means in October we will see her skin glow and watch as she assumes the position of arms stretched out, head tilted back and her body is absorbed in an energy flow and replaced with someone else. That scene's going to look mighty damn odd if we don't actually see that someone else at the end of it.
I mean, they can come back and film a scene with the new Doctor once they're cast and insert it into the episode well before it airs. That's not unprecedented and indeed is basically what Tennant did for his scene in Parting of the Ways. And indeed, t
hat's what you mistakenly claimed was done last year with Dan's scene after Revolution of the Daleks.