1. Fur is completely unnecessary, especially with the many fur substitutes available today that were not available in 1970.
2. Wearing old fur today conveys that fur is okay, and drives the manufacture and purchase of new fur.
3. The manufacture and purchase of new fur is leading many species, especially small cats, toward extinction. Many fur manufacturers purchase not just from fur farms, but from trappers. Trapping is almost entirely unregulated in most states of the U.S.
4. Skinning animals alive for the fur trade, which is still done in many places (especially China, where most American fur comes from) is barbaric, worse even than what is done in the factory farm system. Concepts of animal cruelty have evolved since 1970: while skinning an animal alive may not have been considered excessive cruelty in 1970, it is acknowledged to be so now.
5. Wearing fur on the supposed basis it strikes a blow against PC is a hypocritical form of opportune libertarianism, not terribly different from the guy who tattoos a swastika on his forehead and tries to pass himself off as a free speech martyr.