rejects a position merely on the grounds that someone who argues for it has
The conclusion does reject a position (these critics are wrong). Is the author saying that critics have an ulterior motive when it comes to saying that sentimentality detracts from aesthetic value. The author is saying that their taste for movies is affected by the fact that they are overexposed to sentimental movies, just as someone who always ate garlic would have a different 'palate' from overexposure to that flavor in their diet. An ulterior motive suggests that someone is explicitly seeming to want one thing, but secretly they want something else. I find this answer pretty tempting, because I think we could stretch this conversation into saying, "critics are saying sentimentality detracts from aesthetic value, as though they are simply worried about helping artists create things of optimal aesthetic value, when their real / secondary motive is that they're hoping less-sentimental movies get made, so that they can have a change of pace." We're just adding more story here than we need to, to match up the correct answer with the paragraph. There's no reason we have to think of this situation as, "The critics know what they're doing. They're pretending like sentimentality is bad so that they will get what they want: less-sentimental movies." We could just as easily think of this as, "Critics' taste in movies has been warped by their overexposure to certain types of movies. They genuinely don't like sentimentality anymore. They don't realize it's just a quirk that a compulsive movie-watcher has. They assume it's valid aesthetic observations to say what they're saying." For example, someone who usually has garlic in their food and then has some cuisine that doesn't use garlic might genuinely think that garlic was ruining the flavor of the food they were eating. In other words, it's speculative to say that the critics have an ulterior motive, when they could just have skewed tastes and be speaking to their primary motive.