I think it is possible to leave smoothly, but it depends on one's reasons for leaving. If one "just gets worn out", is isolating oneself, just fades out like that, answers blah-blah-blah-I'll see what I can do etc., then after a while they don't want to waste any more time on you. Every week I was asked by the Service Overseer to join him in the field work, every week something came up, and then the questions came once a month, now every second month etc. If there is a generation change in the congregation, new ones moving in, old friends leaving, people not wanting to leave the JWs can be so much left on their own that they are forgotten and so in fact leave. Having friends, much to do at work - one can slide out smoothly.
But, when it comes to questions, doubts, organizational procedures, legalism, elder arrangement, 607, you name it - no way!