The reality is that very few people in the world are ever going to fall for the Watchtower flavor of crazy, most people can recognize stupid when they see it. When a religion has the lowest retention rate of any, and are growing slower than the population, they are never going to hurt that many people. Granted, even one is to many, but when you look at the number of new people recruited into the religion it really is just a tiny, tiny fraction of the earth's population. You would save more people working on global climate change or world hunger.
It's truly tragic that we can't get more out, but that is the nature of cults. You just can't save everyone, at some point there is a responsibility of each person to research the religion they are in.