Yeah, they need some slaves, but not all of them. Above all, they need those slaves to be obedient. So, the plan seems to be: cull most and put the rest under strict digital control. Say, reduce population by some 90% and make everyone else into a literal biorobot without any free will to speak of, thus dividing human race into two absolutely incompatible castes - almost like two separate species - small in numbers, but very powerful ruler caste and larger in numbers, but totally controlled slave caste.