This led me to thinking: why would a Protestant or evangelical categorically deny the Assumption of Our Lady into heaven at the time of her death?
Depends upon the ones so designating themselves “protestant”, and their position on death.
For instance, Martin Luther (protestant) stated:
“Salomon judgeth that the dead are a sleepe, and feele nothing at all. For the dead lye there accompting neyther dayes nor yeares, but when they are awaked, they shall seeme to haue slept scarce one minute.”
- An Exposition of Salomon’s Booke, called Ecclesiastes or the Preacher, 1573, folio 151v.
“But we Christians, who have been redeemed from all this through the precious blood of God’s Son, should train and accustom ourselves in faith to despise death and regard it as a deep, strong sweet sleep; to consider the coffin as nothing other than our Lord Jesus’ bosom or Paradise, the grave as nothing other than a soft couch of ease or rest. As verily, before God, it truely is just this; for he testifies, John 11:11: Lazarus, our friend sleeps; Matthew 9:24: The maiden is not dead, she sleeps. Thus too, St. Paul in 1 Corinthians 15, removes from sight all hateful aspects of death as related to our mortal body and brings forward nothing but charming and joyful aspects of the promised life. He says there [vv.42ff]: It is sown in corruption and will rise in incorruption; it is sown in dishonour (that is, a hateful, shameful form) and will rise in glory; it is sown in weakness and will rise in strength; it is sown in natural body and will rise a spiritual body.”
- Christian Song Latin and German, for Use at Funerals," 1542, Works of Luther (1932), vol. 6, pp.287,288
“Thus after death the soul goes to its bedchamber and to its peace, and while it is sleeping it does not realise its sleep, and God preserves indeed the awakening soul. God is able to awake Elijah, Moses, and others, and so control them, so that they will live. But how can that be ? That we do not know; we satisfy ourselves with the example of bodily sleep, and with what God says: it is a sleep, as rest, and a peace. He who sleeps naturally knows nothing of that which happens in his neighbor’s house; and nevertheless he still is living, even though, contrary to the nature of life, he is unconscious in his sleep. Exactly the same will happen also in that life, but in another and a better way.”
-“Auslegung des ersten Buches Mose,” in Schriften, vol.1, cols. 1759, 1760
Regarding Luther’s position Archdeacon Francis Blackburne of Cleveland; rector of Richmond states in his “Short Historical View of the Controversy Concerning an Intermediate State” of 1765 :
“Luther espoused the doctrine of the sleep of the soul, upon a Scripture foundation, and then made use of it as a confutation of purgatory and saint worship, and continued in that belief to the last moment in his life.” page 14.
And so, Martin Luther’s ‘sola scriptura’ position; Mary died (1 Cor. 15:22*), rests in the grave, awaiting her resurrection at the last trump.
I am not a Lutheran, but I am only citing Luther’s theological protestant position and reasoning.