The 'preaching work' identifies a particular period of time -- not a religious group. We know that the preaching work had a first century fulfillment, but where in Jesus words do we find evidence that there will be a secondary fulfillment?
Also, when a Witness goes to a door and the householder already has a Bible and a basic knowledge of Christianity, is that due to the preaching work of Jehovah's Witnesses? Obviously not. It is a grand work only in their minds.