Money, money, money!
Pringle1
What is the objection to a PPL/FI teaching a CPL holder? The CPL holder will be trained on their FIC to teach student pilots, something they haven't yet done. Their FIC instructor (ATPL, CPL or PPL) will have a knowledge and qualification that they do not. That is to instruct/teach a pilot for a licence or rating. Neither of which is covered in CPL knowledge or flight test.
On any FIC a presumption is made that the basic skill and knowledge is held. The course is to use those skills to teach others. Should you have an ATPL, CPL or PPL the FIC therefore remains the same.
Is there an argument that could support a view, held by too many, that those such as BEAgle and many, many others - who may not wish to a have a commercial licence, should not teach on an FIC course. The commercial carriage of passengers is not what is being taught.
The FIC minimum knowledge, just the same, should be set to ensure a professional knowledge level of the basics of flight, navigation and the atmosphere. The CPL studies, however, are much wider than that and it follows that much of it is superflous to need.