The laws of tznius, as they are applied to clothing and commonly understood by many in the "frummer" parts of the community, seem to be composed of two related rules:
1. A strict definition on what parts of the female body have to be covered. Collarbones, knees, elbows, etc.
2. An ideal that a woman should not draw attention to herself. The skintight evening gown that may cover her collarbones, elbows, knees, et al, is still no good if it leaves little of her figure to the imagination. The garment may be "kosher" from a covering point of view, but it still attracts attention to the wearer and is therefore forbidden.
However, it should be noted that whether or not an outfit is risque (and attractive) often depends on the surrounding culture. What was considered scandalous a hundred years ago might not even be shocking by today's standards. I don't think you would have seen anyone in Victorian England wearing a bikini at the beach -- and yet, today, it's considered normal beach attire* and is not shocking at all to most people.
I think that if you took all the fashions in contemporary America and graded them in terms of how much they reveal and how likely they are to find acceptence as "normal" within society, you'd find that they probably fall into a bell curve. The most attractive (and/or revealing) outfits would be at the right end of the scale. You'd probably find burquas at the left end. The curve would be the percentage of the population who felt that the particular outfit was in good taste. As you got closer to the average, the percentage of approvers would continue to rise. As you went further out to the sides, the number of approvers would fall.
Tznius, would, in essence, say that only outfits that fall within a certain middle region of the curve would qualify as tznius. Too far to the left and you're attacting attention for being too dowdy (think about the comments that the Beit Shemesh burqua lady was getting -- even before the more serious allegations came to light) and too far to the right and, well... you're just not tznius anymore. In the middle of all this is the average -- the golden mean which would be the norm. So, if you took all clothing in a given society and rated them on a scale of 1 to 100 for attractiveness (with 50 as the average -- the height of the curve), you'd find some outfits (like a burqua) rated at 1 or 2 and some (like a very revealing bikini) to be a 95 or 100. A reasonable rule might be that in order for an outfit to be considered compliant with the rules of tznius, it must fall into a certain range -- say 30 to 70. Anything over 70 is too attractive while anything under 30 is just so ugly/unusual that it draws attention.
Now, let's pretend, for a moment, that Orthodox Jews are the only people who exist on the planet -- or, barring that, that Orthodox Jews live in completely enclosed environments where they will never see a non-Orthodox Jew.
Since Orthodox Jews are the only ones in this society, the (assuming that no one will willfully violate tznius standards) only clothing that will exist is that which falls within that portion of the curve which is acceptable. In theory, the average (the top of the bell curve) will remain the same -- the only thing that will change is that the extreme portions of the curve (to the left and right) will disappear -- as those fashions will be outlawed. So, in our Torah compliant society, only outfits rated 30 to 70 would exist.
The problem here, however, is that attractiveness (as opposed to objective rules about body parts that must be covered) is relative to the society. Therefore, in a society where only clothing 30 to 70 exists (and you'd better believe that the "70" outfits will be more popular than the "30" outfits), the outfit rated 70 now becomes too attractive. Men will start to stare at the outfits rated 60 to 70 (human nature being what it is) and soon those outfits will come under fire as well as being too attractive. Eventually, those outfits, too, are banned -- pushing more and more people to some center where everyone dresses virtually alike.
However, we don't live in such a world. We live in a world where there are people who are not commanded to keep the mitzvah of tznius. We live in a world where bikinis, strapless gowns, showing cleavage, etc. are not uncommon. In other words, having such fashions in our society prevents the mitzvah of tznius from regressing into some nightmare where everyone has to dress identically all the time -- a condition that is so restrictive that, in a free society such as ours, would probably lead to more women dropping out of observance of the mitzvah than keeping it.
In short, I think you can make the case and say that risque fashions save tznius from falling into obsolescence.
* And it works the other way too. In ancient Greece, athletes used to compete in the nude. I think most people, even today, woudl find that shocking.