Some of the points noted (cost, for example, and the work required to learn to fly) aren't necessarily particularly new. I've been thinking about this lately as I hear more and more reasons thrown out for the decline in pilot population and I have a feeling that there's one underlying problem: the romance has gone out of flying. Flying used to be a glamorous activity, with the highly skilled, highly respected, well-paid airline captain at the top of the pyramid. That glamour ran all the way through aviation, from the 747 down to the J-3 Cub, and made it seem worth the time, money and effort to get into the skies.
All of the reasons quoted herein have contributed, of course. Horrible over-regulation rather takes some of the fun out of things, as does expensive 100LL, and the fact that you face ever stronger security even to get into the podunk strip where that J-3 is hangared. Most of us flying now probably still maintain that sliver of romance in our minds (admit it, how many times have you all watched One Six Right?) and it keeps us going. Every time I leave the ground I have my own personal "I can't believe I'm doing this moment" and I hope I can convince just one person that they should feel the same way.
Anybody that tries to sit down and rationally run the numbers is probably going to have their aircraft up for sale within a matter of hours. I'm going to keep doing this until the FAA physically chains me to the ground.