Do we use the phrase "is not so much ... as ..." when comparing opposites?
The phrase "is not so much ... as ..." is an idiomatic expression and common sentence structure. It is not used to make something negative, but it is used for comparisons. Authors use this expression to convey how one thing is of more interest than another. For example,
"Reform is not so much an option as a necessity."
"College is not so much an end as a beginning."
So yes, very often the two things being compared will be opposites. These sentences convey an idea that something is more important than something else. That something else needs to be the focus or that something else should take the precedent. So when you see this phrasing, you should remember that there is a comparison taking place, and it often indicates opposites.