I don't agree. She found her redemption, her friends, and her raison d'etre all as a human. In a way, casting off her pony life was the best thing she did. I think Sunset is right where she needs to be, in the human world, as not only is everything she has now is there, but she's their resident expert on Equestrian magic, and without her, I think the human analogues of the Mane 6 would be stumbling about in the dark trying to figure out how to stop their magical foes (I mean, it was because of Sunset they beat the Dazzlings, Midnight Sparkle, and got Twilight on board with using her magic, which ultimately helped to stop Gloriosa).



Basically, she burned all her bridges when she left, and while she could go back and say something directly to Celestia and maybe see her family, she has no real incentive to go back, especially to stay for good, as her true home is in the human realm now. Besides, anything she could say to Celestia directly she could possibly do through the magic journal at this point.