MLS-12D & KCDW
There isn't a shortage of new planes; in fact quite a number of interesting and "sexy" designs have appeared recently, note the 4-seater DA40 for example and too many 2-seater composites for me to keep track of.
The problem is the lack of students. As KCDW absolutely rightly points out, the decline being in the USA too shows that it isn't anything much to do with cost (whereas the UK flight training industry keeps laboriously pretending it is).
So, what keeps people away? Why are most of the pilots who still hang in there middle aged and often a lot older than that? There is a vast number of men, and some women too, in their 20s and 30s who earn £40k-£60k but few of them choose to learn to fly.
It's another subject, but basically anyone with the money to pursue a significantly time/money-consuming leisure activity is looking to buy into a useful social scene. And there isn't any, in GA, really...
The state of the aircraft is several stages down the road from the primary problem.