I apologize in advance for the length of this post, but the debate over 606/7
BCE and 587 BCE is not new. In 1904
Russell received the following letter, which was published in the October 1 Watchtower
on page 296 (Reprint page 3436).
“Dear Sir, since you have changed your views respecting Gentile Times let me suggest the possibility of still another error. You count the seventy years Babylonian captivity of the Jews as beginning with the overthrow of Zedekiah, Judah’s last king, but I notice that “Bishop Usher’s Chronology,” given in the margins of our Common Version Bibles and based on “Ptolemy’s Canon,” begins that seventy-year period nineteen years earlier-namely, in the first year of Nebuchadnezzar, when he took captive Daniel and other prominent Jews and laid the Jews’ country under tribute. Now if this, the common reckoning, be correct, it would make the Times of the Gentiles to begin nineteen years later than you estimate, namely, in B. C. 587, instead of B. C. 606; and this in turn would make those times end nineteen years later than you have reckoned - in October, A. D. 1933, instead of October, 1914. What do you say to this? Are you humble enough to acknowledge that I have struck some new light, and that you and all DAWN readers have been “all wrong,” walking in darkness?”
Russell’s reply was;
“We reply that there are too many ifs in the proposition, and that they are all abundantly contradicted by facts and Scripture and are therefore not worthy [of] the slightest consideration. (1) The brother errs in supposing that we have changed our view of “Gentile Times.” Those “times” or years are 2520, with a definite beginning in B. C. 606, and a definite ending, A. D. 1914. We know of no reason for changing a figure: to do so would spoil the harmonies and parallels so conspicuous between the Jewish and Gospel ages.”
So the reason Russell stuck with 606 BCE was because, to do otherwise, would “spoil the harmonies and parallels between the Jewish and Gospel ages”. Harmonies that the Watchtower has long since discarded. So right from the start, 606/7 BCE was chosen because it supported Watchtower doctrine, the facts relating to the year have always been irrelevant.