Let's not forget that Linux is just a component of the entire GNU/Linux (I know, I know) system. The accompanying software has grown considerably: GNOME, KDE, browsers, various programming languages and environments, utilities, networking tools, etc. all siphon the interest of a young developer into other areas. Linux is, for all its good and bad sides, stable and feature-full. It's not very tasty to hack on unless you have specific interest in OS internals. So while Linux might be losing the influx of fresh blood, there's still lots of energy around it.