If I was going to start a flying school, I would get a fleet of diesel DA40s, without a question.
Not for ab initio PPL's - that would be pretty foolish. Teaching how to use the mixture/carb heat and running checks is a basic part of the PPL. How are you going to teach your students that in a FADEC machine ?
For hours building hire, then your talking. But for basic PPL training, maybe not.
I don't hold a PPL (hoping to start very soon though) and I wouldn't feel comfortable spending my money flying in an aircraft that doesn't teach me some of the fundamentals to flying other, more regular aircraft. Considering most schools/clubs still use Cessnas and Pipers on Avgas for their hire aircraft, then I would be at a disadvantage.