We learn from the bible that the characters that preached doomsday messages, such as Lot, Noah, etc all got to see the result of the message they preached, we studied that the final judgement from God was seen by the preacher as well as the people that heard and ignored the message.
My point is this, it would have been ridiculous for Noah to start preaching to people, put the fear of God in all the listeners that the Flood is coming, build the ark and then as a result, nothing happens to them. Even if the flood would have happened later, let's say 50 years after Noah and all the people that heard him have died from natural causes. There would have been NO point for Noah to have received the command from God to preach at that time, if no one would have lived to see it the flood.
God would have simply waited to start the preaching work at a later time so that the ones hearing the message would also get to see the fulfillment.
I think the same should apply today, why start preaching in the 1870's, scare all those people into obedience and then not one of them see the end? All those people, perhaps in the millions are all dead, the ones that preached and the ones that heard and ignored, all dead from natural causes. If God supported the JW's, wouldn't he have acted while they were still alive, like the bible examples we studied?