Why does a desktop need to be ultra thin?
Why does a desktop need to be anything but?
Screen size is a limit; can't make that any shorter or narrower than the user will tolerate. Those of us getting the 27"er won't settle for less than a huge screen.
That aside, what hard limits on size are there? Most users are quite satisfied with something that's little more than a big-screen notebook. Few will ever upgrade any components; RAM being by far the most likely, and storage well served by USB or networked devices.
In the interest of persuading customers into the future of computing, obsolete peripherals get dropped. Galled me that my last computer (abandoned just 2 months ago) wasted desk space on floppy drives, parallel ports, empty drive bays, and other never-used interfaces. Take away everything I didn't need, and what I could do without most of the time, and the whole thing would just about disappear from my desk. And so

is doing just that: eliminating what the vast majority of their customers don't want and don't use. Insofar as I do use optical media, at this point it's just to rip it so I can deep-storage the space-consuming boxes; once that job is done, leaving a portable DVD drive in a drawer will more than satisfy further needs (just like my 3.5" Floppy USB drive - available if needed, but would rather it not be built into the computer).
My one gripe with the new design is putting the SD card slot in the back, far from the edges. Non-phone/tablet cameras are still common, and their physical media remains the most convenient way to transfer pictures. That said, maybe I'll try the Eye-Fi solution.
Yes, there is a niche which needs real flexibility and/or power. Given they're maybe 1% of the market,

is sensibly content to focus on high profits from the remaining 99%.