Monocock
Without a doubt a C172 is fine for PPL training. For some strange reason nobody has yet come up with a plane which is as robust and widespread (although obviously the latter bit is a circular argument!)
A completely separate matter is what sort of plane is needed to draw in discerning clients (or whatever you want to call them). It is not a Cessna. Something sexy, like a modern composite e.g. a DA40, would do it. But a DA40 isn't as solid as a C172. So it's a catch-22 for a new school: get the old proven iron (and get the same customers who for the most part can' t afford to fly, and who are quite happy with the decrepit state of GA) or get something new (and assuming slick marketing get some wealthier customers but also get some teething troubles, especially if you go for the diesel version).
No easy solution.
Ultimately one is up against the PPL syllabus which is going to make any "modern" punter fall over laughing. The slide rule... the customer probably drove up in a car with GPS in it!
I do see both sides of the argument so let's not go into that again, but the whole PPL training setup ensures that very little can be done to improve matters.