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Canadian Thomist philosopher Charles de Koninck was a big defender of the Assumption. He saw Munificentissimus Deus (1 November 1950), which Pius XII promulgated only a few months after his anti-Modernist and pro-Thomist encyclical Humani Generis (12 August 1950), as contra Cartesian dualism (mind-body dualism),* since the Pope stresses the unity of the body and soul in proclaiming our Blessed Mother was assumed both body and soul into heaven.
*Cartesian dualism says that there are two types of beings, utterly disconnected from each other: "thinking things" (res cogitans) and "extended things" (res extensa); cf. Four Ages of Understanding p. 523 or the more popular account, ch. 2 of Philosophy of Mind by Dr. Ed Feser.
From De Koninck's "The Immaculate Conception and the Divine Motherhood, Assumption and Coredemption":
THE SIMULTANEITY OF MARY’S DEATH AND ASSUMPTION
The person of Mary, being merely human, depends, for its reality, upon the actual union of body and soul. If, then, her soul had been maintained in a state of separation; if, during some course of time, she had been in death, she would have ceased, for that time, to exist as a human being, as a person, and as Mother of God. Such a state would mean that, during some lapse of time, however brief, the grace of Motherhood—the seat of her habitual grace and of all her privileges as Mother of God—would have been withdrawn. Now, “what is bestowed through God’s grace is never withdrawn except through fault.” But Mary, conceived Immaculate and wholly confirmed in the good, could never have committed the slightest of faults. For this reason, we have argued elsewhere that no lapse of time occurred between Mary’s death and Assumption, in which her soul would have been in a state of separation from her body; but that, on the contrary, her resurrection from death took place in the same instant as her death itself.
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There seems to be a misunderstanding about the nature of canonization inherent in the question.
Canonization (of saints) is the process by which any soul is declared officially to be in heaven, but it does not mean that those not yet declared to be in heaven either a) are not, or b) are not worthy of heaven—even if not there yet (e.g. are still living).
The sainthood (state of being worthy to enter heaven at any given time, including being there) of those who are canonized, are normatively inferred by an official investigation by the Church into the reality and recognition of miraculous answers to prayers resultant from invoking the intercession of the given person.
Sainthood does not necessarily begin when one is in heaven. Indeed those who have no remaining temporal debt for forgiven sins (with no purgatory to endure before entering paradise) are already saints. They have made sufficient reparation for the wrongs they did in so doing attaining perfection "in this life" and not by compulsion "in the next" (Mt 12:32) in purgatory. They have not so much as a "penny" remaining (Mt 5:26) to repay God in any sense, since they made it their duty to do so while in earth, by His grace. It is rare for someone to depart this world in such a state, or to find someone so willing to sacrifice comfort in this world! God is always willing (Jude 1:24), but not always the soul (2 Cor 6:1) to whom He provides His help/grace.
Accordingly, Mary, who never knew sin, as His sacred Ark, was a saint from conception and remains so, and did not become a saint only when she entered the glory of her Son:
Psalm 132:8 Arise, O Lord, into thy resting place: thou and the ark, which thou hast sanctified.
† Sancta Maria ora pro nobis! †