I would bet that there are very few teachers who stand regularly for a meaningful amount of time. A change in times and student base dictates that we make some prudent changes in teaching methods as long as basic principle are not violated and introduced at times that can be meaningful to the student and not counter productive. Not all Taijiquan methods advocate or require Post Standing. Still methods are NOT appropriate for all people. But Basics are.
Basic training is the missing part.
Doing lines of Brush Knee or Part horses mane across the training floor accomplishes a lot of the same thing. It also has the advantage of allowing an instructor to correct things that are part of the routine that the student will be doing. Academic schools have a curriculum. You do not get to calculus without first learning arithmetic. I have tracked drop out points over several years and this is where the largest drop out occurs.