SOTV, has pretty much hit it on the head. Teaching is about empathy and an ability to communicate as well as a passion for the subject. It is nothing to do with the qualifications that the candidate holds.
By removing the requirement to hold a CPL we open the field up to more candidates. There are many people holders who have the empathy and experience to teach but can't get a CPL for medical reasons for example. This does not stop them from becoming a good Instructor.
The whole point is that the FI course is to weed out/improve the candidates and teach them to teach. When I did my own Instructor course there were others on their who had rusty skills, mostly the airline jocks who were not used to PFL's and stalls in small aircraft who were brushed up.
I would expect the kids doing an FI straight from training to have acceptable skills most of the time, although again having seen a few of them you wonder.
But the point that I keep trying to make is that re-opening the opportunities for PPL to teach again will allow us to draw more consistency back into teaching.