Many JWs, my wife included when she was still in and I was mentally out, cannot bring themselves to believe that their loving God Jehovah is going to kill virtually everyone (except maybe those who are Bible Studies) on earth at Armageddon. They would much rather believe that Jehovah will individually judge "the hearts" of humanity and that hopefully tons of non-JWs will be allowed to survive.
If that's the case, the preaching work is NOT a "live saving" effort. The only way it can be is if the all-loving Jehovah kills 99.9% of the planet's population at Armageddon.