You have asked multiple questions so I have multiple answers.
I think the threat of Armageddon right around the corner causes many to stay JWs, if they believe it. I don't know if it causes people to join, since reasons for joining a cult are complex and varied and often involve social relationships.
As far as being good out of fear of punishment or promise of reward, that may motivate some. However, I think that part of maturing as a human being is to get beyond that. That is appropriate for children. It shouldn't be for adults, and certainly not for anyone who is trying to have a spiritual life. I am reminded of the Muslim saint Rabia of Basra. The story goes that she walked the streets of her town with a flaming torch in one hand and a pail of water in the other. She said she was going to use the fire to burn down paradise and the water to douse the flames of hell. She wanted to love God out of a pure heart, not for fear or gain.
Whether one believes in God or not, having a sense of values and following them contributes to the quality of life of those who do it.