If you look at someone who is overweight and assume that aspect of their appearance means they are less disciplined than a thin person, you may be making a major mistake. A lot of it does come down to genetics. I have always eaten whatever and however much I felt like and I've always been slim. Even when I wasn't working out at all, my metabolism kept the pounds off. That's just how my body works - staying slender requires no effort at all. Someone else with a different metabolism might have to exercise fantastic self-discipline in their diet and exercise just to keep themselves from becoming morbidly obese.
Technique is a different issue. You would hope that an instructor would be able to move well enough to illustrate good technique. That's not to say an instructor who is too old and injured to perform all the moves can't be a good coach. If they have the eye to see what you need in order to improve and the communication skills to get their points across, they can still be very helpful. In those cases, though, it is a good idea to have an assistant instructor who can actually demonstrate the moves.
Technique is a different issue. You would hope that an instructor would be able to move well enough to illustrate good technique. That's not to say an instructor who is too old and injured to perform all the moves can't be a good coach. If they have the eye to see what you need in order to improve and the communication skills to get their points across, they can still be very helpful. In those cases, though, it is a good idea to have an assistant instructor who can actually demonstrate the moves.