The flaw in Sprong's and Crossan's approach to hermeneutics is three-fold. First, they recognize Matthew is making a valid point, but they assume he simply made up the details to illustrate that point. Second, they fail to correctly understand (and apply) the basic meaning of the Greek word, παραβολή, "parable:"
The word ‘parable’ is simply the English form of a quite common Greek word (parabolē) which in ordinary Greek usage meant the putting of one thing alongside another by way of comparison or illustration. Aristotle, for example, defines the word as meaning ‘comparison’ or ‘analogy’. (Rhet. 11, xx, 2-4). But in the Greek Bible the meaning of the word is affected by the meaning of the Hebrew word māšhāl Aramaic: məethel) which it was used to translate; and as a māšhāl has a number of uses, so in biblical Greek does the word ‘parable’.1
Third, they fail to consider the entire New Testament which clearly states parables may involve that which is real:
Which is a parable (παραβολὴ) of the time present: according to which gifts and sacrifices are offered, which can not, as to the conscience, make him perfect that serveth, only in meats and in drinks (Hebrews 9:9 DRA)
Accounting that God is able to raise up even from the dead. Whereupon also he received him for a parable (παραβολῇ). (Hebrews 11:19 DRA)
Both passages use παραβολῇ, the same word used to describe Jesus' teaching in the Gospels. Yet the writer is not using something fictitious to illustrate their point. Rather, they understand how real events and real objects shaped, guided, and recorded by God, may be understood and compared to real objects and events in the life of Jesus to demonstrate truth from God's perspective. These are what the writer shares with the reader. It is possible to examine this as a Christian midrash, however that is misleading if by "midrash" one means events which were contrived to illustrate a point.
In other words, just because the events have cultural or extra-Biblical significance, does not mean Matthew borrowed the story in order to make a point.
The basic definition of parable is placing two things side-by-side to illustrate a central idea. The two things can be similar or antithetical:
Then two robbers were crucified with him, one on the right and one on the left:

39 One of the criminals who were hanged railed at him, saying, “Are you not the Christ? Save yourself and us!” 40 But the other rebuked him, saying, “Do you not fear God, since you are under the same sentence of condemnation? 41 And we indeed justly, for we are receiving the due reward of our deeds; but this man has done nothing wrong.” 42 And he said, “Jesus, remember me when you come into your kingdom.” 43 And he said to him, “Truly, I say to you, today you will be with me in paradise.” (Luke 23)
This image illustrates the "parable" of the crucifixion. Two convicted sinners are placed side-by-side with Jesus in the center. One is repentant; the other is not. As John Breck says:
It is perhaps significant that the terms "parable" and "parabola" are derived from the same Greek expression for "juxtaposition" with implied parallelism. Like the geometric figure, the chiastically shaped parable juxtaposes elements in a curve, proceeding from a central point. While the extremities reflect each other as mirror images, they never meet; rather each extends to infinity. In chiastic patterns strophe I and strophe II are dissimilar as well as similar. The movement from the initial element to its "prime" reflection (e.g. from A to A') is almost invariably one of heightening. This means that II is never a simple repetition of I, but rather complements it by taking it a step further. Yet the elements of both I and II derive their ultimate meaning from the "parabolic center."2
Salvation is real because it is based on real events.
1. D.E. Nineham, The Gospel of St. Mark, The Seabury Press, 1963 p. 126
2. John Breck, The Shape of Biblical Language: Chiasmus in the Scriptures and Beyond, St. Vladimir's Seminary Press, 1991, p. 181