One of the justifications I recall was that the prophecies were not completely fulfilled when they were uttered. For instance, Isaiah's prophecies regarding the Restoration's Vegetarian Lions and Nonbiting Cobras were not fulfilled when the Jews returned from Babylon. Lions ate babies, and cobras still poisoned you. So the Society's reasoning ran thus, "Since God's word cannot fail, and all that he said didn't occur then, then there must be a future fulfillment". And hopefully this one will actually come completely true.
Only it didn't. And it won't. There is NO reason to allow for a future fulfillment, neither stated nor implied. It was just a guess, a lucky guess that has somehow survived hundreds of years... BS
CZAR