Mesh import should not harm the fashion attachment business at all. A sculpt is a nice compact little mesh before it ever becomes the confused little animal that SL makes it. The general economy will take a hit while people switch from one to the other but a collada mesh is much more user friendly to the amateur and hobbyist than sculpt maps are. We can use Blender, Truspace or Wings to build and map a mesh with greater accuracy and less time. The high end sculpt business right now is dominated by people who use Maya and ZBrush. Call me a cynic but I am willing to bet that most of those people are using pirate copies of those two VERY expensive pieces of software. Even 3DMax and Softimage have a huge learning curve which has held back a great many SL content creators. Mesh import will open that market to more people.
The down side is yes there will be a great many issues with pirated content from the existing 3D community. Since I belong to that community and I am active in it I can say the idea makes me shudder a little. However, that community features high poly meshes and modeling techniques which I think will not satisfactorily export as a collada mesh.
I am sure that LL will include a vertice or polygon limit as well. Just as nearly every other modifiable online venue does. Whether that is the Sims, The Movies, Quake, etc. There is no reason to maintain the use of sculpts once mesh import becomes available but there is also no reason why that should hurt anyone’s business. Every sculpt in existence probably as an OBJ or a 3DS mesh version as well. Some creators who sell sculpts in building kits already offer them in OBJ format as well. The people who build their business entirely out of such kits maybe hurting at first but the benefits far out weigh the "suffering" in that regard.
The sky isn't falling; it is just starting to rain soup. Remember to bring a bucket instead of a spoon.