The reason the the current crop of training aircraft are used despite being nigh on 40 years old is simply they are the most cost effective aircraft to operate.
Despite all the composite wonder stuff and sexy cirruses this still hasn't changed.
Sadly no one seems to be capable of designing what is effectively a airframe as strong as the C150/2 /172, PA28/38 and mate it with a 10% ethanol sipping EFI rotax. But as soon as they do I suspect the current training fleet will be replaced within a few years.
The bit that amazes me is that no one has been able to do this. The market must be huge.