Languages evolve. Even English.
True, the KJV is in Modern English (i.e. English from Shakespearan times). Prior to that you have middle English (Chaucer) until the Great Vowel Shift in England, completed in roughly 1550.
The KJV and Shakespeare are in Early Modern English
Middle English must not be understandable to you, here's a sample from Chaucer:
Whan that Aprill, with his shoures soote
The droghte of March hath perced to the roote
And bathed every veyne in swich licour,
Of which vertu engendred is the flour;
Whan Zephirus eek with his sweete breeth
Inspired hath in every holt and heeth
The tendre croppes, and the yonge sonne
Hath in the Ram his halfe cours yronne,
And smale foweles maken melodye,
That slepen al the nyght with open y
e (So priketh hem Nature in hir corages);
Thanne longen folk to goon on pilgrimages
And palmeres for to seken straunge strondes
To ferne halwes, kowthe in sondry londes;
And specially from every shires ende
Of Engelond, to Caunterbury they wende,
The hooly blisful martir for to seke
That hem hath holpen, whan that they were seeke
And Middle English dates from 1066 after Old English merged with French (that's why you have two terms for the same thing -- boeuf or beef from the French and cow from the Old English)
Old English is even more difficult for a Modern English speaker to fathom:
here's a sample of the Lord's Prayer in Old English
Fæder ūre þū þe eart on heofonum,
Sī þīn nama ġehālgod.
Tōbecume þīn rīċe,
And this lasted just 500 years, as did Middle english. Modern English -- most folks can't understand Shakespeare which was early modern English and English is evolving further. Modern English woudl be replaced with post-modern English (example -- most folks don't know the difference between can and may among many other words leading to a further simplification of the language, perhaps the perfect tenses will be removed), why for most people today reading Shakespeare (Modern English) is difficult -- so works in Modern English like Shakespeare would be as intelligible in 400 years as Chaucer is to a person today...