"Why isn't she using her seal that's clearly still available on her forehead to regenerate from the damage?"
Better yet, there is the option of parrying or grappling Sasuke's arm to prevent it from reaching Naruto. Itachi and Kakashi have already repelled Chidori using just their hands. Why can't Sakura do the same against an exhausted Sasuke with strength levels that I would hope are close to a perfect statistical 5 by this point?
It sounds tragic, but since we already know the outcome, courtesy of Pierrot's movie, the atmosphere and severity of Sakura's fatal wound might be lost somewhat, in my opinion.