Surely I wasn't just the only one to think this. The Doctor could have sacrificed one of his lives for Clara and then just regenerate. Also, since Ashildr is immortal, couldn't she have just taken the death sentence instead?
ANSWER
As explained in the episode, after Clara took the death sentence, neither Me nor the Doctor could transfer it again. Me was working with a creature called the Shade, and they had, as Me called it, a deal. Presumably Rigsy was never meant to die, the whole thing was orchestrated to bring the Doctor to Me so she could teleport him to wherever he is now. Clara, being reckless, intervened and took the sentence for herself, removing Me from her own deal with the Shade and entirely unable to remove the death sentence.