Imagine if as a developer you only have to worry about a single platform?Actually, this was one of my first reaction as a developer. I developed and maintain a scientific software package (treebank search) that I maintain for Windows (>= XP), OS X (>= 10.7), and Linux (latest Ubuntu LTS).
My first reaction after hearing that Mavericks is free was: great, if most people will upgrade, I am freed of supporting the legacy OS X versions.
Hopefully they can bring things to the point where, like iOS, practically everyone will run the latest version. Of course, this would require smooth transitions.