We could spend all day analysing the goals of different persuasive speakers, but the main thing to take from this page is that the speaker has to choose a goal, and some goals are harder to achieve than others.
For example, Darth Vader might have had more success if his goal had just been to persuade Luke to believe that the dark side might not be so bad, and maybe consider joining at a later date. Instead, he pushed Luke too far.
In contrast, the article about genetic screening chose a relatively modest goal of making the audience feel good about genetic screening. It might have struggled if it had tried to convince the audience to actually get up, go out, and get screened for genetic illnesses themselves.