The author doesn’t have to assume anything about individuals from earlier eras. The argument concerns only modern literature and its effects. The author does not conclude that modern literature is worse for people and society than earlier literature.

Necessary, because if it is NOT to anyone’s advantage that they be concerned with contributing to societal good, then we have no reason to think being UNconcerned with societal good would damage someone. If there’s no advantage from such concern, lack of the concern would not lead to a disadvantage.

The author’s reasoning never involves a requirement that someone believes their own society is better than most other societies. Notice that the premise does not involve a comparison to other societies.

“Aesthetic merit” (the artistic quality of the work) is irrelevant to the argument’s reasoning.

Not necessary, because the author’s conclusion doesn’t depend on a comparison to earlier eras. Although the argument does mention that modern literature is different from earlier eras for the purpose of establishing that the modern literature involves sympathetic treatment of certain protagonists, the conclusion doesn’t assert anything about how modern literature compares to earlier literature.