I dont think any of them are now sincere believers , however they have invested so much time and effort into promoting the JW brand ,pride/loss of face is preventing them from ever giving it up.
Also
Its a lifestyle they and their wives have become accustomed to and they are reluctant to give that up given their age and what else could they do now.
And so they will keep the the farce up as long as they can knowing their is no Jehovah ,no Jesus that is ever going to ask an account from them.