Jehovah Witnesses how come the overthrow of 'Christendom never happened?

1889 "Remember that the forty years' Jewish Harvest ended October A.D. 69, and was followed by the complete overthrow of that nation; and that likewise the forty years of the Gospel age harvest will end October, 1914, and that likewise the overthrow of 'Christendom,' so-called, must be expected to immediately follow." (Studies in the Scriptures, Vol. 2, p. 245)