In an ironic way, the WTBTS needs apostates.
Their whole worldview is framed as a battle. Satan of course is their invisible enemy, just as Jehovah and Jesus are their invisible God and his Son.
That being said, and all JW rhetoric to the side, JWs are obsessed with visible things. They need a visible enemy to confirm their beliefs, whether it be a persecuting government or better yet us "apostates."
What would their struggle look like without any opposers?
What is the sound of one hand clapping?