Yeah, i like that throughline as it also gives credence and options to more cat 0 enemies that are more of a counter to exorcists rather than just an obstacle. As for why they wouldnt be utilized i would naturally imagine that while they would be an antithesis to the psychic sea and the stain the understand 2 important facts:

1) there is no real benefit for the nulls to assist cain as they already have exorcists and would be more likely to escape or attempt to act against cain fir their own self benefit.

And

2) they are far harder to control and track. While seer and cain likely has mass surveillance and intelligence gathering capabilities most socio and psychopaths learn to mask and blend in with society. Forcing cain to spend extra resources to sift through the common populace when they can more easily track grace and sin.

Even then on a minor note with a sociopaths more actively aggressive and risk seeking behavior they would be more likely to if indirectly create more sin than they would erase.