This is an interesting discussion. In my view, everyone is a little off the mark. Really what is needed is a completely separate and discrete licence purely for instructors. It would not depend on having CPL or ATPL knowledge, since there is a lot of material in those ground exams which is not relevant to instructing. Instead it would contain a very large core of teaching techniques and development of presentation skills, combining the relevant parts of the ground school material with a proper teaching qualification.
There would be no route from this licence to an airline licence as it would be purely a teaching qualification. In my view this is the only way forward to establish the serious career instructor as a true professional not an hours builder. Hours building by instructing should be completely abolished. As one post stated, we would not allow the holder of O levels to return and teach at a secondary school. I am constantly disappointed by the total inability of FIC candidates to spell even basic words such as aileron or longitudinal, to write a well-constructed sentence or even to write in a straight line on a white board.
If the route to instructing was only by way of a "teaching diploma" we would weed out all of the hours builders and perhaps begin to gain credibility within the aviation community. Such an enhancement of status might even lead to increased rewards (heaven forbid!).