As per the Preface of the 1611 KJ, and the comments "To the Reader"
Those opponents of the KJVOnly position like to present the following from the Preface of the 1611 KJV
"Now to the later we answere; that wee doe not deny, nay wee affirme and avow, that the very meanest translation of the Bible in English, set foorth by men of our profession (for wee have seene none of theirs of the whole Bible as yet) containeth the word of God, nay, is the word of God."
The purpose of this excerpt is to assert that the translators of the KJ said that every Bible of the past (at that time) was the word of God, but never asserted that their work was to produce the one perfect Bible.
They did say that all previous Bibles were the word of God, but they did not say that their purpose was not to translate a one that was any better than the rest.
Here is what they said as to their purpose ...
"But it is high time to leave them, and to shew in briefe what wee proposed to our selves, and what course we held in this our perusall and survay of the Bible. Truly (good Christian Reader) wee never thought from the beginning, that we should neede to make a new Translation, nor yet to make of a bad one a good one, (for then the imputation of Sixtus had bene true in some sort, that our people had bene fed with gall of Dragons in stead of wine, with whey in stead of milke

but to make a good one better, or out of many good ones, one principall good one, not justly to be excepted against; that hath bene our indeavour, that our marke."
Notice:
1) "Truly (good Christian Reader) wee never thought from the beginning, that we should neede to make a new Translation"
They did not set out to make a "new Translation".
2) "nor yet to make of a bad one a good one"
It was not there purpose to make a bad one better.
3) "(for then the imputation of Sixtus had bene true in some sort, that our people had bene fed with gall of Dragons in stead of wine, with whey in stead of milke
This is a parentheses that defines why the previous two points were not the purpose of the translators. If Sixtus had imputed some of his beliefs as to the truth of Scripture; there would have been (at least in some degree) the necessity of having one, (or both) of the previous purposes.
4) "but to make a good one better"
The translators now state ... "but" a conjunction used to introduce a phrase or clause contrasting with what has already been mentioned.
The contrasting statement is that the previous things were NOT the purpose; BUT these ARE the purpose
.
"to make a good one better".
Until that time there had been several good Reformation Bibles. However, the unique thing about Bibles such as the Bishops Bible (produced by the Church of England ... Episcopalian); and the Geneva Bible (produced by Reformers (Calvinists) in Switzerland), is that they were translated in a manner that emphasized their beliefs.
King James ordered them to work together to translate a single Bible to be used for all.
Hence, the purpose of the KJ translators was to "to make a good one better".
5) "or out of many good ones, one principall good one"
The KJ translators then add, an additional possible conclusion ... out of many good ones (the previous Reformation Bibles), "one principal good one".
"PRINCIPAL
most important, consequential, or influential : chief <the principal ingredient> <the region's principal city>"
Merriam Webster 2019
Wow, the most important, or influential ... I'm okay with that!
6) "not justly to be excepted against; that hath bene our indeavour, that our marke."
Not "justly" to be excepted against. The KJ translators literally stated that those who would try to have an "exception against" (reason to be against this translation), would have no "just" reason to have their objections to it. Why? Because, there was no single group that monopolized its translation: it was the combined work of Christian's, being led of the Holy Spirit.