Would it really be, though?Killing off future beardy Doctor will be a cheat...
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.
Except for approximately one scene in The Two Doctors - and even if they did such a thing, which they might, (or tried making him human and mortal for a the midseason cliffhanger) it'd get retconned as soon as Matt Smith decides to hang up his fez or stetson, and the 12th Doctor is cast.