Ein paar Gedanken...
So, first off, I can't tell from your post if you're talking about the computer or the OS that's running it. You keep vacillating which makes it truly impossible to know for certain what you mean. Therefore, I'll address both.
First, the UI design dates to the late 1970s and Xerox PARC. It's been iterated so many times since then just by Apple, let alone everyone else who's come along and take a whack (or three!) at it. The reason Apple kept it all throughout the Classic Mac OS days, and through the transition and into Mac OS X, along with NeXT using it themselves, and why Microsoft kinda-sorta implemented it in Windows, and why most Linux-environment desktops use it, is because of what others here have said: it just freakin' works. Period.
And on the subject of Linux desktops, most of them (from the most stripped down to the the most elaborate) use nearly the exact same style of UI as Classic Mac OS / Mac OS X. Arguably KDE is the most Windows-like, but nevertheless they're all relatively similar. The popularity of the classic desktop UI metaphor was amply demonstrated when the Gnome Project decided to "tablet UI" their desktop interface, and caused one of the greatest schisms in memory within the Linux community. "Thanks to" the Gnome Project, we now have Cinnamon (yay Clement Lefebvre!) and MATE, etc.
You're right to point out this is not by any means a zero-sum game.
Your points on general concepts remaining around being a shared thing within and outside of the computer world are well taken, and I think particularly apt. However, how exactly is the iPad a new product category? It's a tablet.