Wicker Man.
Award of any licence requires demonstration of skills and knowledge. I would agree that the skills side can be taught by anyone who has passed an Instructor Test subsequent to the FI course. Depth of knowledge is a different thing and it's important to be able to discuss aviation topics at a higher level than the PPL syllabus requires, should the situation arise. We are not talking about driving a car or a truck here - these are purely manual skills. Flying is a little more cerebral. I feel my school metaphor was more appropriate than those you quote.
I was recently asked by a student to explain coriolis effect. How many instructors who have not gone beyond the PPL syllabus would be able to do that?