The problem with using fir in fashion is whether animals are treated ethically or not. We live in a world that uses things up until they are gone. Oil, there is no conversation going that suggests we conserve what oil reserves we have left, only the question of how long will it last and what will we do when it is gone. The only difference with furs is that we are talking about living organisms.
Over millions of years a huge assortment of organisms developed and interacted and created life on earth as we know it today. The most important key point when decided to where a real fur or a fake fur is this, without every organism on the planet today, life on earth is imbalanced. What this means is that we could change the balance of life as we know it on the planet earth by harvesting a species to extinction. Describing to you how that works will take more than this short blog entry but the truth is that all organisms fill a niche and do a certain duty. for instance a mink may control a certain population of rodents. Without the mink around, the rodents grow out of control in a population explosion. These rodents may carry a disease such as a virus that causes death in humans (yes it is true, not a made up story).
So, eradicating a species from the wild could me detrimental not only to that species but to other species as well, including your own. Oh, you say you would only purchase furs from farm raised animals. Consider the conditions on the farms from which you buy. Some farms in order to be profitable will take no consideration for the animals which they eventually kill for their skins alone. Not a very healthy system.
The next time you consider buying a fur, understand what you are choosing and what you are spreading.