When Jesus spoke of the "end" he was talking not about Armageddon but the "END of the gentile times." He was speaking to Jews and they were obsessed with when they would be restored to their homeland again. They were very conscious they were outside of God's favor and understood when God's anger rested against them they would be restored to their homeland and become a nation again.
So the end of the "age" of the "end of the system of things" to them, was the current system of things that had them in bondage, thus they longed for the "end" to come.
So the preaching of the "good news" worldwide has a specific fulfillment in relation to November 30, 1947, when the official "end of the gentile times" occurred. Just before the end would occur, the "good news" which was to be preached by JWs, would have been spread worldwide. To emphasize that, early in 1947, president N.H. Knorr went on a worldwide speaking tour. Thus he literally preached the good news worldwide, that is, throughout Christendom nations, and then the end came on November 30, 1947.
Anyway, that is the actual fulfillment of that prophecy. The worldwide speaking tour by NH Knorr in early 1947 that saw the "end" come on November 30, 1947. Recount in Luke a reference to the "end of the apointed times of the nations" showing the context of the Jewish focus on when they would be restored to their homeland. That is the "end of the system of things" = the "end of the appointed times of the nations" that they were talking about. Not Armageddon.
So. That prophecy was, indeed, fulfilled.