Assalamu-alaikum wa rahamatullahi wa barakatuhu

E H Palmer, as early as 1880, recognized the unique style of the Qur'an. But he seem to have been wavering between two thoughts. He writes in the Introduction to his translation of the Qur'an:
That the best of Arab writers has never succeeded in producing anything equal in merit to the Qur'an itself is not surprising. In the first place, they have agreed before-hand that it is unapproachable, and they have adopted its style as the perfect standard; any deviation from it therefore must of necessity be a defect. Again, with them this style is not spontaneous as with Muhammad and his contemporaries, but is as artificial as though Englishmen should still continue to follow Chaucer as their model, in spite of the changes which their language has undergone. With the Prophet, the style was natural, and the words were those in every-day ordinary life, while with the later Arabic authors the style is imitative and the ancient words are introduced as a literary embellishment. The natural consequence is that their attempts look laboured and unreal by the side of his impromptu and forcible eloquence.[7]
The famous Arabist from University of Oxford, Hamilton Gibb was open upon about the style of the Qur'an. In his words:
...the Meccans still demanded of him a miracle, and with remarkable boldness and self confidence Mohammad appealed as a supreme confirmation of his mission to the Koran itself. Like all Arabs they were the connoisseurs of language and rhetoric. Well, then if the Koran were his own composition other men could rival it. Let them produce ten verses like it. If they could not (and it is obvious that they could not), then let them accept the Koran as an outstanding evident miracle.[8]
And in some other place, talking about the Prophet(P) and the Qur'an, he states:
Though, to be sure, the question of the literary merit is one not to be judged on a priori grounds but in relation to the genius of Arabic language; and no man in fifteen hundred years has ever played on that deep-toned instrument with such power, such boldness, and such range of emotional effect as Mohammad did.[9]
As a literary monument the Koran thus stands by itself, a production unique to the Arabic literature, having neither forerunners nor successors in its own idiom. Muslims of all ages are united in proclaiming the inimitability not only of its contents but also of its style..... and in forcing the High Arabic idiom into the expression of new ranges of thought the Koran develops a bold and strikingly effective rhetorical prose in which all the resources of syntactical modulation are exploited with great freedom and originality.[10]
On the influence of the Qur'an on Arabic literature Gibb says:
The influence of the Koran on the development of Arabic Literature has been incalculable, and exerted in many directions. Its ideas, its language, its rhymes pervade all subsequent literary works in greater or lesser measure. Its specific linguistic features were not emulated, either in the chancery prose of the next century or in the later prose writings, but it was at least partly due to the flexibility imparted by the Koran to the High Arabic idiom that the former could be so rapidly developed and adjusted to the new needs of the imperial government and an expanding society.[11]
As the Qur'an itself says:
And if ye are in doubt as to what We have revealed from time to time to Our servant, then produce a Sura like thereunto; and call your witnesses or helpers (If there are any) besides Allah, if your (doubts) are true. But if ye cannot- and of a surety ye cannot- then fear the Fire whose fuel is men and stones,- which is prepared for those who reject Faith. (Qur'an 2:23-24)
Lastly, the beautiful style of the Qur'an is admired even by the Arab Christians:
The Quran is one of the world's classics which cannot be translated without grave loss. It has a rhythm of peculiar beauty and a cadence that charms the ear. Many Christian Arabs speak of its style with warm admiration, and most Arabists acknowledge its excellence. When it is read aloud or recited it has an almost hypnotic effect that makes the listener indifferent to its sometimes strange syntax and its sometimes, to us, repellent content. It is this quality it possesses of silencing criticism by the sweet music of its language that has given birth to the dogma of its inimitability; indeed it may be affirmed that within the literature of the Arabs, wide and fecund as it is both in poetry and in elevated prose, there is nothing to compare with it.[12]