See, here’s how I would have done Donna’s ‘death’: Donna starts saying, like she does in the show, that she doesn’t want to go back. The Doctor tells her that even without her memories and without him, she can still be a hero and indeed was in the parallel Turn Left verse. The only difference is that he won’t be able to help her, she’ll have to help herself, but he knows she can do it. Bearing that in mind, Donna agrees to the mind wipe.
There we go, still sad, but less icky and nooneisworthanythingwithoutthedoctor-y.