Denim jeans were originally worn not so much as a fashion

I received a pm from Archit143 about this question. The first thing I will say is --- I am not impressed with the quality of the question. Yes, isolates an important idiom, which I will discuss. But all five answers have that awkward phrase "for their being practical work clothes" --- that abomination would never appear on the real GMAT.

There seems to be a great deal of confusion about the idiom in play here. Let me distinguish two closely related idioms
1) not A but B
2) not so much A as B / not as much A as B
Both of those are 100% correct on the GMAT. The first one is a black & white distinction --- whatever we are talking about is not at all A, but rather, only B. If I say "thing is not A, but B" that implies the statement "thing is A" is totally false and the statement "thing is B" is totally true.
The other idiom connotes a difference of degrees, shades of gray ----- both statements, "thing is A" and "thing is B" are partially true, but the latter is correct more of the time than is the former. In saying "thing is not so much A as B", I am saying, thing is really both in some sense, but the relatively emphasis is on B.

If I say: "I listen to not rock music but classical music." --- this means that 100% of my listening is classical, and 0% is rock. It's very absolute, very black-and-white.

If I say: "I listen to not so much rock music as classical music." ---- this means, yes, I do listen to both, but far more frequently to classical --- it may be a 30%/70%, or a 5%/95% split --- we don't know the exact ratio --- we just know that it's mostly classical and there's some amount of rock music in there as well. There's no absolute separation.

Here, even apart from the other grammatical considerations, it would be illogical to say
Denim jeans were originally worn not as a fashion statement but as practical work clothes.
We would be saying --- when blue jeans were introduced, you could scour the Earth looking for folks who wore them, and not a single person attached any fashion significance to them. That's crazy. Of course, as practical as blue jeans were, someone somewhere must have thought of them as fashionable as well. All it takes is one hot person showing off her or his curves in a pair of jeans and BAM it's a fashion statement. The 0%/100% split just doesn't make sense in this context.

Some of the answer answer also mix and match these two idioms which you can't do
not A as B
not as much A but B
not so much A but B
Answers choices (B) & (C) & (D) & (E) fall into this category, so they are all wrong.

(E) is an absolutely nightmare. That string of words "but as for them being" will never be correct in any GMAT SC problem.

The only possible right answer is (A), although I consider even this a poor sentence that the GMAT would likely consider incorrect. I will suggest as a superior version:
Denim jeans were originally worn not so much as a fashion statement as practical work clothes

Does all this make sense?

Mike