Most stay in, but perhaps that’s beginning to change. One of the things that cause people to leave sometimes is if they move to a different city and don’t make friends in the new congregation. Or if they get disfellowshipped and don’t got back. Or they pursue a new interest such as education or a career or business or hobby that requires a lot of time and they become immersed.
Coming to that, I do know a couple of sisters who refused to go to their assigned congregation. They attended another congregation for a while then became inactive. So I do wonder if there will be an impact from all the KH reorganisation that is going on.
Plus there is the phenomenon that very elderly JWs tend to disconnect, mainly because JWs are extremely poor at including older people in their community when they stop engaging in the preaching work.