Sherman means that the translators did not believe they were creating a "perfect" translation, but only striving to create the best one that they could. In the Preface, they wrote that they were not aiming for a "new translation" or to make "of a bad one a good one," but rather, "to make a good one better." They also said that the "very meanest" translation of Scripture into English by "men of their profession" (that is, by men that had actually be trained in Hebrew and Greek) contained and was the Word of God. In other words, they saw nothing terribly special about the KJV that should make it the one-and-only translation choice of the people, nothing that should make it the "perfect" Bible of the English language, and they absolutely understood their limitations as human translators. I think the translators of the KJV would have lauded with extreme enthusiasm the continued efforts to refine and improve translations in the modern day.
As for your question about aionios in earlier English translations, unfortunately, as far as I know, no. From about the end of the third century (when Koine fell out of usage), most translation work treated New Testament Greek as though it were, more or less, a variation of classical Greek. Koine was largely lost as a dialect until the collection of the papyri in the nineteenth century (documents in Koine from various authors, including everything from works of literature to grocery lists). In classical Greek, aionios becomes inundated with the idea of "eternity," so when these translators saw this word in Greek, they immediately thought "eternal," and with relatively few exceptions, that was how it was translated, and has continued to be translated even till today. The tide of scholarship is slowly changing, but 1700 years' worth of linguistic tradition is not easily shaken, especially when linguistic tradition is so bound up with theological tradition.