I disagree that one cannot learn on their own. All MA came from somewhere and someone had to devise each one in the first place. There is not an endless regression of teachers. At some point in the mists of time a person or group of people came up with the principles of Tai Chi and applied them. This occurred with all the MA’s. The process of discovery included a period of trial and error wherein movements and principles were modified, improved, discarded and added. What allowed the originators and modifiers to create and modify was insight and study into the principles and applications of the movements in conjunction with trial and error and real world application. If one studies and considers deeply what they are learning and consults references such as videos, books and the opinions of others they will gain benefit from their study. Learning something is better than learning nothing.
Miyamoto Musashi, one of the greatest swordsmen in the history of Japan, had no formal instruction. He was laughed at by his opponent prior to his first duel at aged 13, a man he soon killed! It may be argued that originators and modifiers were exceptional individuals. Well, who is to say you are not an exceptional individual? Exceptional individuals forge their own path motivated by a drive to know and understand and to do it their own way! The limitations we have are the limitations we choose to accept. Do not let others argue for your own limitations. Their limitations are their own to overcome do not accept their burden as your own! You are not teaching yourself skydiving here. You will not kill yourself or endanger your safety by learning Tai Chi from a video!
It cannot be denied that having an instructor can be beneficial and may perhaps be preferable, but one must also have a GOOD and COMPETENT instructor. Novices are in no position to determine who is a good and competent instructor. It may take years to discover one is learning from an inadequate instructor. Anyone studying under an inadequate instructor will learn poor and inadequate skills and principles which they will one day need to unlearn. How is this different than learning inadequately from a video? There is no difference! Inadequate is inadequate! Many times inadequate is in the eye of the beholder anyway!
Serious study of MA is a life time process. Masters continue to gain insights as they teach and study. It may be argued that inadequate instruction is worse than no instruction. But since the student is in no position to perceive inadequate instruction he has no idea if he is learning inadequate principles and skills in the first place. If he is, he must unlearn those poor skills! Since inadequate is in the eye of the beholder it is best to start somewhere and learn as you go while gaining knowledge from alternate sources and personal study.
So to the argument that you may learn bad habits, I would reply: all new students have bad habits they must overcome. Long time practitioners have bad habits they must overcome. Masters have bad habits they must overcome. No one is perfect. Improvement is constant. Learn to discern your own inadequacies through self observation, study and insight. How do you think Masters discover their own short comings? They figure it out for themselves!
To insist that the only way to learn is from an instructor is an overused and blindly repeated principle passed on from instructor to student without any real questioning of its validity. While it may be well meaning is not an absolute!
Do not choose videos from only one source, use a variety. Lean to observe the movements closely. Think about what you are learning. Teach yourself to observe your own movements and introspect into your mind to gain insight into the movements and principles. This is how the originators made their discoveries. Many things that are learned cannot be taught by another, they must be perceived through insight. Study books as well. I suggest “Tai Chi Classics” translated by Waysun Liao and there are many other valuable sources. Yang Jwing Ming is a good source for many topics.
Forge your own path, learn what is of interest to you and do not allow naysayers to negatively influence you. If and when you are able to, or become so inclined, study from an instructor. When you do empty your cup and learn that instructor’s method. Then integrate what you have learned from the instructor into your own knowledge base.
Good Luck!