I agree 100% with Fuji Abound.
I was not after mandatory USE of GPS; it would be its inclusion in the PPL (which itself is highly unlikely) that would result in mandatory INSTALLATION in a proportion of training aircraft. Mandatory USE of anything is not enforceable (company manuals in commercial ops aside) and that's why no law prohibits any use of GPS...
Mandatory installation of GPS in all aircraft would do a lot more good than mandatory Mode S (and at a similar cost)
HWD - flying might have been a "simple activity" in 1930 but it isn't today.