好

You're wrong. Principles are only there to teach you. Once you have understood what they bring, you can transgress all principles. You will be able to issue power from any part of the body and in absolutely any position: seated, lied down, on your knees, with wobbling legs, knees, leaned on the side, etc... It really won't matter anymore because you will be able to connect and join no matter your position. The way you think will limit your Taichi. You are too much of an iconoclastic Taijiquan scholar. Once you have understood and integrated the teachings of the principles and the form. You have to forget them, and just move naturally. And this style was created by the first grandmaster of the three internal arts. It really was not created for show. Sun style integrate Bagua footwork, Xing Yi legs and waist, and Taiji soft body relaxation. You really have no idea what you're talking about. For you knees not going beyond your feet is not a hard principle, but a soft one. The importance is the feeling you have in your feet. If you are fully and properly rooted, it does not matter if your knees goes beyond the tips of your feet because you will be properly connected from the feet to your Dan Tian. We all have different bodies.