I don't think cost is the main problem. While there is no doubt that
participation in just about any type of flying activity is shrinking (and
has been for quite a long time, at least in the USA), I believe that there
are other factors at work that are responsible for this. Cost is certainly
a factor, but I don't think that it is the primary one.

One factor that makes flying a bit unattractive is that a simple PPL
does not have a lot of utility and is not the equivalent of a driving
license in the sky. Sometimes it is sold that way though. When I took
my first intro flight in Northern CA, the instructor tried to sell me
the idea that once I get my PPL, I would be free as a bird and could
fly to on a whim to Tahoe for skiing! I think that the PPL kind of
flying is more of a sport than anything else like mountaineering or
skydiving or even gliding and maybe needs to be marketed as such. It
just so happens that on some nice days it can be used as a means of
transport but this cannot be the main reason for getting a PPL.