I agree that OS X has been partly responsible for converting people away from Windows, but equally responsible is a growing Linux community. Since Linux came out, it has been slowly gaining some of Microsoft's desktop market share, especially since Gnome and KDE have gotten more polished and Ubuntu made its number 1 bug Microsoft.
I was a bit disappointed to see no mention of open source software, because it is not Google Docs that is replacing MS Office, but Open Office. GIMP has been slowly replacing Photoshop since it came out, not Snipshot. Likewise with Firefox overtaking Internet Explorer, Thunderbird being the first to stand in for Outlook, and gcc has been the compiler of choice for years.
In summary, I agree that Microsoft is on the decline and that we are seeing software shift to the Internet, but I never think the desktop will go away. The number of desktops running Windows and other Microsoft software, however, is going to continue to decline. It's one thing to offer free software on the Internet alongside Google ads, it's another entirely to be able to install entirely free, ad-free software with no strings attached, which is an opportunity that the open source community has afforded us.