Evil persons promote those assumptions to deflect opposition to sin. But in fact, the whole purpose of material life is to overcome sin, and negativism is how it is done. So most, but not all positivism, obstructs the process. Therefore, they must be promoted separately, which means at different times. Positivism is needed for accomplishing complex things with groups.
Negativism is needed for correcting errors or corruptions.
It is not good to mix negativism with complex interactions, because it is disruptive. It forces everyone to get embroiled in the inanity of a few. Also, corrupted persons will use squabbles as a disruption tactic. They justify or promote corruption through fights. By getting a battle going, they do or say things which would not normally be acceptable.