Learning Tai chi from a video is like learning Chinese from a Chinese movie. Sure you might get the sounds right in places, you might even be able to quote some scenes from the movie verbatim - but you won't know what you're saying, or whether you're pronouncing it right until you have someone who knows what they'r talking about listen and correct.
I personally do not like learning languages through movies; however, I have heard of more than one person who has claimed to have learned English in that way.
I have to say that I actually mostly agree with what you are saying; however consider two things. Some people live in remote areas far from qualified teachers or have little money to travel or pay for listens. Is it really better that such people do nothing rather than study with a commercial DVD and begin to learn a form? I believe I have met people who have had limited exposure to a teacher and yet have studied our form through the DVD. To me, it seems that they can indeed progress to a certain point before they will hit a wall.
If we are talking about coming close to mastering a significant portion of Tai Chi. Then I agree that this cannot be done by video. I would also agree that looking at the typical YouTube videos would not be very productive, since there is usually only very limited instruction.
My own recommendation is that a commercial DVD would be fine to begin with, but that it would be important to visit a teacher, even if only a few times a year, to get corrections. When there was certain instruction that I wanted, that is the method I have adopted. Even now, I live about an hour away from most of my practice partners and/or students, and so they or I have to travel significantly to learn and practice the things we want to study.
As for things like correcting a straight back, I think that many students who have studied for 5-10 years or more still receive such basic corrections on a fairly frequent basis. Doing Tai Chi is easy. Doing it well is not.