That's a good question about designers setting themselves up to fail. I believe the top fashion designers don't intend for their runway fashions to be worn by the masses. However, most of them probably do expect to be able to modify their runway looks to appeal to a broader audience, and to have those modified designs sold through the top department stores. There are probably other designers who truly do design for a small segment of the population, and who don't care if the average person never wears their clothing.
It doesn't take long for popular designs to be copied and mass produced. This happens much faster now than it did even a few years ago. Ironically, I suppose the more "wearable" designs are more likely to be plagiarized. It's a back handed compliment, I guess.
And, of course, you may have general trends in design that don't necessarily belong to a particular designer. Those general trends, rather than specific garment designs, can be modified much more easily to appeal to the general public. Neon + Neutral, for instance, can be interpreted many different ways, and most consumers are likely to find at least one interpretation of the trend that they like.
I'm sure Angie or some of the others here can expound on all this, or correct me if I'm wrong.
