The Old Days?
Why? To teach someone to drive you need a full driving licence, to teach HGV or PSV you need to hold those licences. To say you need a CPL to instruct for the PPL is like saying you need to be able to drive a formula one car before you can instruct on a Mini Metro. The old system of PPL instructors worked very well before for a very long time ( not all instructors were ex-military ) before the BCPL came along. I would prefer proven experience and common sense to exams.