Good post but I think that it is missing something. Must electric guitars made today are built to last for decades. Even a $150 Yamaha Pacifica is an extremely good quality instrument.
That means that each passing year the market is being flooded with second hand guitars that are good quality and in good conditions. I've owned countless guitars and basses and I have gotten almost all of them second hand, the same with most of my musician friends. I would say that most electric guitars are being bought second hand so the real numbers of guitars out there could be bigger than ever. Asia and South America are full of young players.
Is it true that rock music is not so prominent anymore and that people prefer airy, soft music instead of aggressive sound of the guitar but things have a way of coming back. All it takes is one successful artist or band that brings rock back to the mainstream. As Neil Young sang: "Hey hey, my my, Rock N' Roll can never die... My my, hey hey, Rock N' Roll is here to stay".
Indeed, but this second hand market has always been around and should not lower current sales compared to old ones. As every trend, it will come back. Stronger than ever :)