The problem with the FDS doctrine is that directly after the Faithful and Discreet Slave is mentioned, other parables, such as the 10 virgins and the illustration about the workers and their Talents are viewed as lessons by Watchtower, when the Faithful Slave should really be viewed not as a prophecy but simply as an illustration. The stupidity of this proves true when we read that half hearted explanation on how the part mentioning "if ever that evil slave" now means a warning to the Faithful Slave which should not be taken as a prediction the FDS will turn evil. Utter rubbish.
Watchtower tends to cherrypick Bible verses, and the FDS probably is only to be used in reference to family heads in the largely patriarchal approach Christianity takes.
The Governing Body don't need a Scripture to validate their position. History proves that prophets, although supposedly Divinley appointed, were clearly democratically chosen as such.