This discussion overlooks a few key texts.
Here is a prayer written by St. Athanasius of Alexander (296-373 AD), considered the most strident defender of orthodox Christianity against the threat of Arianism:
It becomes you to be mindful of us, as you stand near Him who granted you all graces, for you are the Mother of God and our Queen. Help us for the sake of the King, the Lord God and Master who was born of you. For this reason, you are called full of grace. Remember us, most holy Virgin, and bestow on us gifts from the riches of your graces, Virgin full of graces.
If Athanasius believed that Mary was just like any other woman when she died, what's he praying like this for?
St. Ephraim the Syrian (306-373 in Syria and Edessa) wrote sermons and hymns that were widely used throughout the Christian world. His works were translated into Greek, Armenian, Coptic, Slavonic, and other languages and are still used in many churches. St. Jerome (347-420) wrote that churches would use Ephraim's hymns after the scripture readings during their masses. Here's what Ephraim wrote in his 4th hymn on the Nativity of Christ:
Of a sudden the handmaid became the King's daughter in Thee, Thou Son of the King. Lo, the meanest in the house of David, by reason of Thee, Thou Son of David, lo, a daughter of earth hath attained unto Heaven by the Heavenly One!
In his 11th hymn on the Nativity, he wrote (titled loosely "I shall not be jealous of my son"):
The Son of the Most High came and dwelt in me, and I became His Mother; and as by a second birth I brought Him forth so did He bring me forth by the second birth, because He put His Mother's garments on, she clothed her body with His glory.
Yes, both Athanasius and Ephraim wrote in the 3rd-4th centuries AD. However, it's impossible that both of them generated new dogma out of whole cloth with not a ripple in the documentation of the time. Both of them were embedded in well-known and highly-documented locations and an introduction of entirely new Mariology in those locations would have been baffling and controversial.
Athanasius' writings were widely read throughout his time and later centuries. Ephraim's hymns and sermons were translated and spread throughout the Eastern Church, and the Latin Jerome attests to churches using his hymns in liturgy.
These figures' writings, well before the Council of Chalcedon's definition of Mary as theotokos in 450 AD, show that the belief in Mary's fate after her earthly life was both widespread and well accepted. While that clearly can't say that the belief in the Assumption/Dormition of Mary was present at the beginning of Christianity, it does strongly document that it was a widespread and well-accepted belief at/near the very beginning of Christian "officialdom" within the Roman and Edessa worlds.