Aquinas did not create a nature/grace dichotomy. Read de Lubac, Pinckaers, O’Meara, etc. Heck, even read Copleston. There’s no such thing as “pure nature”, most Thomas scholars agree. Ergo, I shall resort to proof-text mode but just this one:
For the affection of charity, which is the inclination of grace, is not less orderly than the natural appetite, which is the inclination of nature, for both inclinations flow from Divine wisdom. (ST II-II, 26. a.6)
If you didn’t catch that I’ll paraphrase: Both natural inclinations and inclinations of grace come from God. As Pinckaers points out it was the prerogative of Ockham to change the meaning of “nature” by separating nature from freedom. He had the idea that true freedom was not the ability to choose what makes one more human but the capacity to choose not to choose. Thus human nature lost those gracious natural inclinations and instead became subject to the will. Nature was no longer seen as including gracious elements but actually became a nuisance to free will. Posse peccare was better than non posse peccare. This was part of the new “science.” The objective eclipsed the subjective.
The harmony between humanity and nature was destroyed by a freedom that claimed to be “indifferent” to nature and defined itself as “non-nature.” The consideration of the nature and spiritual spontaneity of the human person was banished from the horizons of thought. (Servais Pinckaers, The Sources of Christian Ethics, p. 333)
For Aquinas the will of man is naturally attracted to the good which finds its end in God. “… all persons do share in the same last end, because we are all seeking happiness, understood as the ultimate and complete fulfillment of all that we are seeking in our lives … there is objectively only one thing which can provide us with this happiness, and that is God Himself.” (Jean Porter, “Right Reason and the Love of God”, in The Theology of Thomas Aquinas, p. 172) All men participate in God’s Eternal Law through the innate first principles of the Natural Law. I think we tend to forget that all men were created in his image. And yes, that does apply to more than the material nature. Grace does not destroy nature precisely because nature is graciously predisposed to grace.