I'm familiar with CAFE standards but US politicians don't seem to have much intent on changing them.
And it is not economically unviable, it is just not as profitable because every other country produces and sells properly sized cars with a profit (except maybe Australia).
The problem with these structures is usually small marine life - they tend to need frequent maintenance to remove barnacles and whatnot.
Having your smooth walls and turbines lined with barnacles tends to impact performance significantly.