1. Donna chooses to have her mind wiped.
Donna’s time with the Doctor has made her realise she was special all along. She comes to understand that Donna Noble, the temp from Chiswick, is no less important than the DoctorDonna, saviour of the universe. Although she’s still crying and scared out of her mind, she tells the Doctor she wants to go back because she’s still got work to do. She needs to buy her gramps a better telescope and the nice house he’s always deserved. She needs to hug her mum and tell her she loves her, that old cow. She needs to go back and make her dad proud. She’s going to keep saving the world, one small act of kindness and bravery at a time, because she knows that even if she’s not the most important person in creation anymore, if she can be important to just a few people, then that’s all that matters.
2. Donna chooses to die and the Doctor respects her choice.
Donna knows she has lived and loved more in this last, chaotic year than
she ever could in a whole lifetime. She chooses her short,
beautiful, wonderful life over the prospect of a long, empty and
unfulfilling one and decides against having her mind wiped in a glorious affirmation of the Doctor’s words:
‘Some people live more in twenty years than others do in eighty. It’s not the time that matters. It’s the person.’
And the Doctor, although his hearts near rip in two from watching his best friend die in his arms, respects her choice. He understands it shouldn’t be up to him to dictate who lives and who dies and that if Donna wants to die as the woman she’s become, then so be it. In spite of the pain, he takes a measure of comfort from her smiling insistence that even if she had a time machine and could go back and make things right, she’d make the same choices all over again. She wouldn’t trade her time with him for anything in the universe.
Dalek Caan’s prophecy is fulfilled and a child of time dies, the child who never became a soldier but instead stayed gentle and kind, who defeated Davros with her voice, her wit and her laughter, who never wielded a gun and saved all of creation without killing. She dies with no regrets and the Doctor carries on, emboldened by her legacy.
3. Basically by not stripping Donna of her agency and choice and making the mind-wipe meaningful beyond just causing the Doctor angst????