I think that the point that might be overlooked here is that even if a person does "die in this system", they would be resurrected to a life of working toward perfection for 1000 years. There is no guarantee that they will survive the "final unloosing of Satan". They could still be destroyed and lose their everlasting life...thus a JW would say that they still needed Jesus' sacrifice to ultimately gain everlasting life, providing they make it through the 1000 year reign of Christ. They will be called from their graves to work toward perfection and then ultimately face destruction if they fail their test of loyalty.
Isn't that clear as mud??...