This is a question I've thought about too. Most religions have a 'carrot' for their believers, be it heaven, re-incarnation or the JW paradise earth. As we know, this is not a new concept and was used by the ancient Greeks, Romans, etc., to good effect.
This invites the following questions - does this mean that people following a religion are doing it for reasons of self interest/preservation? Or - is their relationship with their God central to their faith and any 'carrots' are viewed as a bonus? As JWs are told continually that Jehovah can read their heart then surely he would see the extent of their selfishness, so in effect you would be spiritually 'rumbled'.
My own view is that you would have hoardes of people, particularly young people who would abandon the JWs, as there would be pain with no gain. From my own observations at my old KH, the majority of people appeared to be attracted to the JWs because this paradise earth would fix all their problems, illnesses etc., that they couldn't cope with in the real world. Take this cure all away and their really isn't much incentive left.
FF