Killing off future beardy Doctor will be a cheat...
Would it really be, though?
I have a friend who was so utterly pissed off about the Doctor's line about 507 regenerations in "The Death of the Doctor" that I swear she was going to spit nails. It wasn't the casual disregard of the "rule of 13" that everyone knows. It's that 507 is
so far away from where we are today that the show will
never reach 507 Doctors. 507 took away an endpoint for her. She wanted a final Doctor, because she wanted a look at what the Doctor will do and what the Doctor will become if he knows that his permanent death is a very real possibility.
Assuming the bearded Doctor is a future Doctor, and not a clone Doctor or a shapeshifter pretending to be the Doctor, killing him would give my friend
exactly what she wants. We have an endpoint -- the eleventh Doctor dies and he doesn't regenerate -- and we know it's going to happen, and what does the Doctor do when faced with that knowledge? That he really and truly is mortal?
That's territory that
Doctor Who hasn't mined before.
If that's the story Moffat wants to tell, I see the obvious out. "Time can be rewritten." Moffat has been setting up that story -- first in "Flesh and Stone," then "The Big Bang," and then "A Christmas Carol." Why keep hammering away at that idea, that time can be rewritten, if he's not going to use it? It's Chekhov's gun on the mantelpiece.
Can the Doctor rewrite his own life and avoid his fate?
If I were Moffat, that's what I'd do. Kill a future Doctor.