Taabsr, I always appreciate your videos. Thanks for posting them.
I think the presumption that most people will eventually use a waist harness is false. As has been said many times on this forum, the choice between waist and seat harnesses is one of personal preference. There are many highly experienced and skilled kiters who use a seat harness. Even if many may use a waist harness after learning, there are a considerable number of kiters who will not.
When I taught kiteboarding, I always had 3 types of harnesses to show my students: waist, traditional seat, and board shorts. We talked about the advantages and disadvantages of each. Many students (windsurfers, climbers, pear-shaped people) gravitated to the seat or board shorts harnesses simply hearing a discussion of the differences. But it's a choice that the student should make for himself or herself, not one that an instructor should pre-determine.
During lessons, for reasons others have already given, my students used a seat harness unless they specifically requested a waist harness. I haven't heard anything in this thread that would change my approach. For a beginner trying to figure out kite control, a seat harness is the better choice.
And for experienced riders who want to hold lots of power, have a long de-power throw to ride big over-powered kites, and boost airstyle tricks, or who may prefer a low hook based on a history of windsurfing or climbing, the seat or board shorts harnesses may be preferred.
In my personal riding, I have used all three types of harnesses. I started with a seat, went to board shorts, then to a waist, and now I'm exclusively using a seat again. A seat suits my riding style and, as others have said, I can ride all day without any core fatigue. I can also keep a seat harness much looser than a waist harness, which facilitates airstyle tricks. There are many advantages to a seat harness, and there are lots of kiters using them at my riding locations.
Bottom line for me is to provide students with information on all types of harnesses. Don't over-complicate lessons; a seat harness during lessons simplifies the learning process. Eventually, each student will make a choice that feels right for him or her.