Granted, all of what you say is true; I am not in any way suggesting that this notion stands anywhere near the center of Orthodox anthropology. Simply trying to locate Patristic sources discussing this so as to perhaps ascertain in what sense they may or may not be harmonized with the tradition of the Church.
I did find a reference in St. Augustine's 'Enchiridion', Chapter 9 (NPNF Translation), online at: http://www.ccel.org/ccel/augustine/enchiridion.chapter9.html
To wit:CHAPTER IX. The Replacement of the Fallen Angels By Elect Men (28-30); The Necessity of Grace (30-32)
28. While some of the angels deserted God in impious pride and were cast into the lowest darkness from the brightness of their heavenly home, the remaining number of the angels persevered in eternal bliss and holiness with God. For these faithful angels were not descended from a single angel, lapsed and damned. Hence, the original evil did not bind them in the fetters of inherited guilt, nor did it hand the whole company over to a deserved punishment, as is the human lot. Instead, when he who became the devil first rose in rebellion with his impious company and was then with them prostrated, the rest of the angels stood fast in pious obedience to the Lord and so received what the others had not had—a sure knowledge of their everlasting security in his unfailing steadfastness.
29. Thus it pleased God, Creator and Governor of the universe, that since the whole multitude of the angels had not perished in this desertion of him, those who had perished would remain forever in perdition, but those who had remained loyal through the revolt should go on rejoicing in the certain knowledge of the bliss forever theirs. From the other part of the rational creation—that is, mankind—although it had perished as a whole through sins and punishments, both original and personal, God had determined that a portion of it would be restored and would fill up the loss which that diabolical disaster had caused in the angelic society. For this is the promise to the saints at the resurrection, that they shall be equal to the angels of God.
I think I can see how the above could be harmonized with Orthodoxy. There is nothing to suggest such a far-fetched notion that God created man for the purpose
of replacing the ranks of fallen angels, nor does it seem to be a statement of anthropology generally. It seems more concerned with eschatology, for as the Lord Himself teaches us '[...] In the resurrection they shall neither marry nor be married; but shall be as the angels of God in heaven.' (Mt. 22:30)
I don't suppose St. Augustine is the only Father to discuss this?
Fr. Sergius Bulgakov alludes to Patristic sources for this in his 'Jacob's Ladder' (pp. 47-49; I quote from pp. 47-48) but, frustratingly, there are no citations given: 'This is connected with a more general question: could not the devastation in the heavens which was produced by the fall of the angels be reflected in the destinies of the earthly world which in the fallen angels lost its guardian angels and received in their stead hostile hordes of demons? An answer to this question is sometimes given in patristic literature in the sense that humankind was created as it were to replenish the void that was formed after the angels' fall (which is why the number of humans, evidently, must correspond to the number of fallen angels, perhaps deducting those who do not participate in this replenishment because they are predestined to the eternal fire prepared for the devil and his angels from the foundation of the world.) The creation of humankind is considered here to be the results of the angels' fall and is consequently in and of itself an ontological accident, the outcome of the shadow which fell on all creation after the angels' fall. If we talk this thought out to the very end it will turn out that there would be no humans if the angels had not fallen. Of course such a strange and barren concept would not correspond either to the sense of the biblical narrative in Genesis 1-2 or to the general fundaments of Christian anthropology and angelology. Although between heaven and earth a positive congruence exists, there remains at the same time an opposition; hence, the replacement of a part of one world by a part of the other world is generally an ontological misunderstanding. [...]'
Fr. Sergius is clearly referring to teachings beyond those of St. Augustine here. I am simply interested in discovering which 'patristic literature' this may refer to.