Yeah, I dunno. Maybe someone else who knows the film better or has seen it recently can help with that.
Nevermind, I think I know exactly why now.....
Dahlia's mothering of Natasha will last forever for AT LEAST 4 reasons:
1. The human soul is, by nature, eternal.
The human soul, being noncorporeal and thus, completely metaphysical by nature, cannot be destroyed by any conceivable thing within the physical realm. Since in death, the soul is completely metaphysical by nature, it exists in a higher order of existence and has transcended the boundaries of the physical realm, therefore the very concepts of death, destruction, aging and even harm itself do not apply. Those that enter the spirit world are no longer subject to death as they are no longer bond by their corporeal forms.
2. The afterlife exists in eternity.
Those that transcend into the spirit world transcend into a higher order of reality in which restrictions such as time and space itself in the physical universe do not exist. When they perish, they enter eternity, or a plane of existence in which time does not exist. Since time does not exist, souls that are already eternal by nature, do not age.
3. Human/Mortal love is never enough to satisfy such a hungry spirit.
Man, being flawed and fallible by nature, will never and can never deliver the most perfect, flawless love enough to fully satisfy another human soul to where they will cross over. And the human soul, being eternal in nature, requires a powerful, eternal, everlasting love from a Higher power to satiate it wholly. Man, being mortal and fallible, can do nothing to fully placate or satisfy a spirit enough to banish it to the netherrealm or deliver it to the afterlife.
4. Natasha's rage/vengeance/sorrow/anguish lasts forever.
Lastly, the fourth and most important reason why Dahlia will need to appease Natasha forever is due to the irrevocably damaged state of Natasha's soul. The little girl died in such a tragic and unexpected way in having no knowledge as to why her mother abandoned her as well as living her entire life neglected by both her mother and the father who didn't protect her, that it gave birth to a powerful rage/sorrow, creating a malevolent spirit from an otherwise innocent child. Once Natasha's soul became corrupted by such powerful negative emotions and circumstances, it not only binded her soul to the physical world, but completely changed her into something else--a monster. And as it is well known in Japanese Mythology that a spirit will not and cannot rest so long as the very emotions that binded them to the realm of Man in first place upon death persist, Natasha will not and cannot cross over because of her anger/sorrow. Short of appeasement from a motherly figure (who herself is also cursed as a result of the sacrifice), Natasha's wrath persists forever. Even after having gained Dahlia's love, the state of her damaged spirit does not change. She remains a potentially vengeful and dangerous spirit. And since the potentiality for causing harm to the living will always be there, her being trapped in limbo will always be. Natasha's wrath lasts for eternity, therefore Dahlia's surrogacy will last forever.
To fully placate/appease Natasha to where she would finally cross over into an existence of true everlasting peace and happiness would require absolute perfect, flawless and infallible love, as well as the complete and utter eternal relinquishment of the negative emotions that bind her soul to the physical world.