I think the question arises out of a dubious translation, and I'm not sure what accounts for the ISV's almost unique offering of "fragmentary" here.1 Even so, there is something here worth probing, although the way the question is framed partially obcures this. The central question (slightly tweaked for clarity) is:
[OP] ...what point is [the author of Hebrews] trying to make about the NT vs the OT in the first verses [of the book]?
In fact, the book of Hebrews begins with the phrase in question. It's first three words are:
Πολυμερῶς καὶ πολυτρόπως...
Polymerōs kai polytropōs...
Since OP has already provided full lexical entries for these key (and resonant) terms, that need not be repeated. It is worth supplementing this with a couple other items that provide some semantic clarity. First, is the brief entry in Moulton & Milligan's (that's the "MM" with which OP's lexical entries conclude) The vocabulary of the Greek Testament Illustrated from the Papyri... (1914), p. 527:
πολυμερῶς ... denotes "in many portions" as distinguished from πολυτρόπως, "in many manners"...
with brief citations of their sources corroborating this claim. This has often been discussed by Hebrews commentators. And the older discussions by, e.g., B.F. Westcott or James Moffat have not really been superseded.2 Westcott offers the most insight via patristic commentary, but Moffat's brief reflection (see his p. 2) on the import of the phrase is worth quoting at length:
The writer does
not mean to exclude variety from the Christian revelation; he
expressly mentions how rich and manysided it was, in 24. Nor
does he suggest that the revelation ἐν προφήταις was inferior
because it was piecemeal and varied. There is a slight suggestion
of the unity and finality of the revelation ἐν υἱῷ, as compared
with the prolonged revelations made through the prophets, the
Son being far more than a prophet; but there is a deeper
suggestion of the unity and continuity of revelation then and
now. Πολυμερῶς καὶ πολυτρόπως really "signalises the variety
and fulness of the Old Testament word of God" (A. B. Davidson). On the
other hand, Christ is God's last word to the world;
revelation in him is complete, final and homogeneous.
As Moffat's comment makes clear, then, the essential contrast with 1:2 between the poly~/poly~ character of revelation through multiple prophets (and at multiple times) ἐν τοῖς προφήταις, with a singular revelation through one "source", "a son" ἐν υἱῷ. The claim here then is not so much one of simplicity over complexity, but singularity as against multiplicity.
Two other considerations reinforce this observation. First is the structure of the "letter" to Hebrews itself. Each of its main sections is designed to contrast the (singular) Son with the variety of precursors in previous ("old") revelation:
| | Hebrews | Contrast/Compare |
| 1. | chs. 1-2 | Jesus v. Angels |
| 2. | 3:1 – 4:13 | Jesus v. Moses, Joshua, Aaron |
| 3. | 4:14 – ch. 10 | Jesus & Melchizedek |
| 4. | chs. 11-13 | Outcome for life of faith |
Of course, some nuance could be added to that rudimentary structure, but it serves to show the essential contrast of Jesus, "the Son", with any of the exemplars that went before.
There is a further parallel to this line of thought in 2 Peter 1:19-21, which provides the same claim in miniature. Peter has been speaking of eyewitnesses at the moment of Jesus' "transfiguration", and goes on to say, as a result:
19 So we have the prophetic word made more sure, to which you do well to pay attention as to a lamp shining in a dark place.... 20 But know this first of all, that no prophecy of Scripture is a matter of one’s own interpretation, 21 for no prophecy was ever made by an act of human will, but men moved by the Holy Spirit spoke from God.
This gives us pretty much the same contrast as Hebrews 1:1-2 -- that is, prophetic revelation of old culminates in the singularity and certainty of revelation to through Jesus.
There still might be something to be said for the superiority of simplification here. This is, after all, part of Jesus' own logic in his instructions on prayer in the Sermon on the Mount (Matthew 6:7-8):
And when you are praying, do not use meaningless repetition as the Gentiles do, for they suppose that they will be heard for their many words....
However, perhaps closer still to the sort of "simplification" in mind in Hebrews 1:1-2 is that asserted by Josephus in identifying only "twenty-two books" of the Jews,3 in a famous passage in Against Apion, 1.37-38:
37 Naturally, then, or rather necessarily – seeing that it is not open to anyone to write of their own accord, nor is there any disagreement present in what is written, but the prophets alone learned, by inspiration from God, what had happened in the distant and most ancient past and recorded plainly events in their own time just as they occurred – 38 among us there are not thousands of books in disagreement and conflict with each other, but only twenty-two books, containing the record of all time, which are rightly trusted. [John Barclay translation]
It is not just the author of the letter to the Hebrews, then, that sees simplification from multiplicity to be something commendable. Of course, Hebrews reduces the "manifold" revelations of antiquity to the singular revelation of the Son, whereas Josephus praises the twenty-two Jewish holy books against "the thousands of books in disagreement and conflict" of the pagans -- but the rhetorical effect is the same.
Although, to be fair, F.W. Farrar, The Epistle of Paul the Apostle to the Hebrews (Cambridge University Press, 1888), p. 52, does opine that:
...[t]he nearest English representative of the word is "fragmentarily," which is not meant as a term of absolute but only of relative disparagement. It has never been God's method to reveal all His relations to mankind at once. He revealed Himself "in many portions."
It is perhaps telling that the New English Bible (1970) had "in fragmentary and varied fashion", but the 1989 revision ("Revised English Bible") changed that to "in many and varied ways".
See B.F. Westcott, The Epistle to the Hebrews: The Greek Texts with Notes and Essays (Macmillan, 1892), pp. 4-5; James Moffat, A critical and exegetical commentary on the Epistle to the Hebrews (ICC; T & T Clark, 1924), pp. 2-3. Modern commentators repeat much of this same information: see, e.g., Paul Ellingworth, The Epistle to the Hebrews (NIGTC; Eerdmans, 1993), p. 91, or Luke Timothy Johnson, Hebrews: A Commentary (NTL; Westminster John Knox Press, 2006), pp. 64-66.
Josephus's "twenty-two books" have been the subject of much discussion. See, for orientation, the comments of Sid Leiman (from Josephus, the Bible, and History, ed. by Louis H. Feldman & Gåohei Hata [Brill, 1989]); also (out of many!), D.L. Christensen, "Josephus and the Twenty-two-book Canon of Sacred Scripture", JETS 29 (1986): 37-46.