Of course it's flawed. Think of it like a court case. The defendent says he wasn't at the liquor store at the time of the holdup, and the defense cites as its only evidence against this a statement made by a witness that is best interpreted in a way that favors of the prosecution. The prosecution, for its part, has blood DNA evidence, fingerprints, video from the security cam, tire treads of the getaway car outside, ballistic matching of shells found at the scene with the defendent's gun, gunshot residue on the defendant's hands, and multiple eyewitnesses, all of which point to the same exact conclusion. Does the alibi hold? Should all the other evidence be cast aside because you found the defense's interpretation of the witness statement somewhat convincing?
If you look at Jeremiah's original prophecy of the seventy years (or the version in the MT, at least), you can see how it is at odds with the Society's interpretation. Chapter 25 is dated to "the fourth year of Jehoiakim son of Josiah, that is to say, the first year of Nebuchadnezzar king of Babylon" (v. 1), and while it does say that "the whole land shall be devastated and reduced to a desert" (v. 11), it is servitude to Babylon that is supposed to last 70 years: "I will summon all the peoples of the north and my servant Nebuchadnezzar king of Babylon and I will bring them against this land and its inhabitants and against all the surrounding nations.... These nations will serve the king of Babylon seventy years. But when the seventy years are fulfilled, I will punish the king of Babylon and his nation, the land of the Babylonians for their guilt" (v. 9-12). The nations surrounding Judah were devastated and subjugated by Nebuchadnezzar for many years prior to the final destruction of Jerusalem, starting with his accession year. In fact, the oracle implies that the 70 years begin around the time the oracle was given: "For twenty-three years, from the thirteenth year of Josiah son of Amon, king of Judah, until today, the word of Yahweh has been addressed to me and I was persistently spoken to you but you have not listened ... So, this is what Yahweh Sabaoth says: Since you have not listened to my words, I am summoning all the clans of the north and my servant Nebuchadnezzar king of Babylon and bring them against this land and its inhabitants and all these surrounding nations" (v. 3, 8-9). It was in the "fourth year of Jehoiakim" that Nebuchadnezzar defeated Egypt and its Syro-Palestinian allies in the Battle of Carchemish (see Jeremiah 46:2). The main point here is that the period of servitude started many years before the destruction of Jerusalem in the reign of Zedekiah. That is inconsistent with the Society's interpretation. Another inconsistency is the statement that dates the end of the 70 years when God "punishes the king of Babylon and his nation". That could only fit the fall of Babylon in 539 BC, not the situation two years later (as the Society has the 70 years running out in 537 BC). Now as our resident WT apologist pseudo-scholar would strenuously point out, the period between 605 and 539 BC amounts to 66 years, not 70 years. That is true. Since "70 years" was a stereotypical formula in such oracles, it is probably the case that Jeremiah's prediction simply used a well-worn formula and history turned out to mostly vindicate his expectation. After all, what we have in ch. 25 is a prophetic expectation, not a chronological datum. Some however have pointed out that the first nations defeated by Babylon were Assyria and Media, whose defeat became complete in 609 BC, thereby making the prophecy fit history exactly. This is technically possible, and indeed the era of Babylon's supremacy was about 70 years, but this explanation fits a little better with 29:10 than 25:11. The main point regardless is that the seventy years must have began many years prior to the final destruction of Jerusalem.
This is confirmed in the pamphlet to the exiles in ch. 27-29. At the beginning of the reign of Zedekiah, Jeremiah proclaimed this message:
"Send word to the king of Edom, the king of Moab, the king of the Ammonites, the king of Tyre, and the king of Sidon, through their envoys accredited to Zedekiah, king of Judah in Jerusalem... Give them the following message, 'Yahweh Sabaoth, the God of Israel, says this: I by my great power and outstretched arm made the earth, man and the animals that are on the earth. And I can give it to whom I please. For the present, I have handed all these countries over to Nebuchadnezzar king of Babylon, my servant; I have even put the wild animals at his service. All the nations will serve him, to his son, and to his grandson, until the time for his own country comes in its turn, when mighty nations and great kings will enslave him. Any nation or kingdom that will not submit to Nebuchadnezzar king of Babylon, and will not bow its neck to the yoke of the king of Babylon, I shall punish with sword, famine, and plague" (v. 3-8).
Clearly, these surrounding nations were already handed over to Nebuchadnezzar by the time of Zedekiah's accession, and they will remain subject to him "to his son, and to his grandson, until the time for his own country comes in its turn, when mighty nations and great kings will enslave him". This is a clear parallel to 25:11-12, where the 70 years corresponds to the three generations mentioned here and the reference to God punishing the king of Babylon in 25:12 corresponds to the enslaving of Babylon by "mighty nations and great kings" here. All of this cannot be reconciled with the Society's interpretation.
Note that this was after the siege of Jerusalem in 597 BC, in which the king carried off both Temple vessels and exiles to Babylon (27:18-22). Jeremiah urged King Zedekiah to "bend your necks to the yoke of the king of Babylon, submit to him and to his people and you will live" (v. 12). Meanwhile, the false prophet Hananiah declared that Yahweh "has broken the yoke of the king of Babylon. In two years' time I will bring back all the vessels of the Temple of Yahweh which Nebuchadnezzar king of Babylon carried off from this place to Babylon. And I will also bring back Jeconiah son of Jehoiakim, king of Judah, and all the exiles of Judah who have gone to Babylon" (28:3-4). Again, the period of servitude, the time when "these nations serve the king of Babylon for seventy years" (25:11), had already begun, for God says that "I have handed all these countries over the Nebuchadnezzar king of Babylon ... all the nations will serve him, to his son, and to his grandson until the time for his own country comes in its turn".
Finally, ch. 29 includes the text of a letter that Jeremiah sent to the exiles already sent to Babylon, and he encourages them to settle down there, to take wives and have sons and daughters and then find wives for their sons (thereby indicating the passage of generations), for they were not going home anytime soon: "Only when the seventy years for Babylon are over, will I visit you and fulfill my promise in your favor by bringing you back to this place" (v. 10). Here Jeremiah refers to the seventy years as a period of time that Yahweh granted to Babylon, the time when "all the nations will serve Nebuchadnezzar king of Babylon, to his son and to his grandson" (cf. 27:7). Notice, again, that in 29:10 the 70 years do not lie in the future, for they are not waiting for the 70 years to begin but for them to be completed. The Watchtower interpretation again goes against the context.
Contrary to the use of this formula in Jeremiah, other books of the OT use the expression in different ways -- each offering an inner-biblical exegesis of Jeremiah. These later interpretations of the Chronicler, Zechariah, and finally the author of the Hebrew apocalypse of Daniel each present novel reinterpretations of the expression and depart from the original Jeremianic conception. Since the Society's general fundamentalist stance towards the Bible assumes that it presents a single viewpoint (whereas it really consists of a library of different books from different viewpoints), it effectively harmonizes the texts by starting with the later interpretations and interpreting Jeremiah in light of them. But these later texts do present divergent understandings of the "70 years" than provided by Jeremiah, although they may well have been influenced by a variant edition of Jeremiah that circulated separately (which possibly represented an earlier version of the prophecy, cf. the Josiah-era oracles of ch. 1-2, 4-6, and which is preserved in the LXX). I have a paper by a biblical scholar on this subject I could email you if you PM me.