Shoebag that's pretty harsh. What martial art do you teach? I teach tai chi chaun myself and I see the same fitness issues in my tai chi students as I do in my skiing students. The difference between tai chi and skiing is that tai chi can greatly improve a persons functional fitness if, and this is a big if, they practice regularly and have good instruction. This can have a very big impact on a persons skiing if they choose to ski or already ski. There are other aspects of tai chi that transfer to skiing as well. You are right ni that many people aren't very physically active and so don't have much to transfer from. then there are people who are active but aren't active in ways that maximize their functional fitness. Both categories need to change something if they want to improve. I believe that both can develop into advanced skiers if they put in the work on and off the snow.
I know it does seem like a harsh opinion, but I try to make it in a constructive way. The reality is that we all have our limitations, and the best we can hope to do is excel within them. I think sports like martial arts bring out the best in people, physically, emotionally and spiritually. My background is in Judo, then Karate, then TKD for the last ten years. I have seen students achieve more than they thought they ever could through training, but at the same time reach the limit of their abilities. For instance, no matter how hard you train, only about five percent of martial artists will ever be able to through a true 540. Modern American "sports karate" often promotes people based on their commitment, dedication and time in rank, rather than their actual skill level. The best you can do is make the most of what you have to be as effective as you can. My left knee dislocates if I pivot on it, so when I spar I only pivot on the right, making me completely one sided. However I use fakes, reverses and combinations to compensate, and do very well. If I tried to spar "correctly", I would loose most of the matches I now win.
I think we need to do the same with skiing and recognize that not everyone has the physical attributes to make perfectly linked and controlled high speed turns. Rather than making that the goal, we can set a more realistic outcome and take a students strengths, weaknesses and limitations into account. Sometimes it just comes down to tactics, ski what you can in the best way you can and accept you limitations rather than fighting them. But to do that you first have to accept that your physiology does provide a ceiling to your progress.