Silly silly Minimus. you framed the question incorrectly. You assume that while the d2d is ineffective, it is a means to find new disciples.
The d2d preaching is to exert coercive control over the r&f. Your spiritual and social standing within the congregation is based upon the number of hours you report.
If they wanted new converts they would do away with the d2d preaching and concentrate on the needs of the the potential converts. That will never happen. With converts comes commitments. Commitments are expensive. The last thing the watchtower real estate development corporation wants to do is spend money helping people.