Literary critic: Samuel Johnson argued that writers should refrain from attributing attractive qualities to immoral characters, since doing so increases the tendency of readers to emulate these characters.
Conclusion
If writers were to follow Johnson's advice, works of fiction would be unrealistic.
i.e. if writers were to refrain from attributing attractive qualities to immoral characters, then works of fiction would be unrealistic.
Evaluate
There isn't really any premise, so it's using the term "argument" pretty loosely in the question stem. We know the first sentence is SJ's argument, because it says, "Samuel Johnson argued that X, since Y". So X was SJ's conclusion and Y was SJ's support.
The rebuttal pivot (but, yet, however) signals to us that the last sentence is the author.
We're trying to prove the claim that,
Goal
So the author must be assuming that,
Or if we want to predict a conditional version of the answer,