Even the younger ones who stay in are often just nominal JWs. They have learned to play the JW system. I've a few teenage / early 20 relatives who are baptised but essentially lead lives that aren't much different to their "worldly" peers. It's all about social networking , playing XBOX / PS4 games , hanging out in pubs.
i think that is an important point. For many it is becoming a cultural religion. They believe in God but don't take the GB or 1914 too seriously. But they may as well put in nominal worship with their friends and family.
The big divide between JWs and Christians is the belief in paradise on earth. It is easy to be a nominal Christian and hope you are worthy of heaven at death. But for a JW they need to hope for armageddon so that they don't die, or are at least non existent in the grave for just a short time. The longer time goes on from 1914, the harder a "soul sleep" based religion will find it to retain members.