Despite its user interface, when it comes to touch and digitizer support Windows 8 is far better than Windows 7. There are many under the hood improvements in handling that kind of input which 7 lacks. People got so focused about the Metro UI they missed what other was done. I understand any attempt to build a tablet with good pen input supports needs Windows 8, not 7.
You may like It or not, but in "tablet" mode - without a mouse and a keyboard - Windows 8 "modern UI" works well enough, as it does in Windows Phone. If you need to build an application using a "clean" working areas with all toolbars, etc. hidden, Windows 8 UI frameworks already works that way and you don't need to reimplement it from scratch while keeping away all the other apps. Sure, this is not way most business application works, but for some, it works, and very well.
It's when you try to use it in a classic desktop environment it really doesn't fit well - while the classic shell is not really good when you need to use it on a tablet - and that was what made "Tablet PCs" a niche product.
I use a Surface 2 Pro as my portable device - and I like its pen input very much, I get rid of paper for quick notes and sketches, but I still have Windows 7 on my desktop too (where I have a Wacom tablet connected, anyway).
Also you may like it or not, but most of the photo/design/etc. business workflow is built upon Macs. Using a Mac ensure a smooth workflow avoiding all those little differences that can get in the way when you're not an IT guy but an "artistic" one , they are willingly to pay for the convenience, as you say. But as you said - "to do proper design, you need a Mac." (That isn't true, BTW, and ceased being true a long, long time ago.)" - today Windows applications are on par with the Mac ones.
Once you're inside the application, and it does what you need - you don't care much about the OS running it.
Unlike other users that may use several different applications at once, those working in sectors like photo editing, design, etc. spend most of their time within a single application. I know some that even keep a second computer at hand for "secondary" stuff like email, browsing, etc, because their expensive workstation with custom calibrated monitors run only the software they used for production. They rarely see the OS.