The author does not assume that moral socialization cannot be changed in the future. The author’s conclusion is solely about the effect moral socialization has had until this point.

The author does not assume that there is no causal relationship between two things. The author’s assumption is that the increase in moral suffering caused by the feelings of guilt/self-loathing in many people is not outweighed by a reduction in suffering in other people.

The author does not conclude that any aspect of how we socialize children should be changed or eliminated. The author’s conclusion is simply a descriptive statement about the effect of moral socialization.

The author thinks that because moral socialization sometimes leads to suffering (by causing many to feel shame), it can’t significantly reduce overall suffering. This is flawed because moral socialization might help reduce suffering in others, thus reducing suffering overall.

The author doesn’t assume that “no one” can have a “positive reaction” to moral socialization. Some people might react positively; as long as this reaction isn’t a reduction in suffering enough to outweigh the increased suffering in many people, this doesn’t hurt the argument.