Nobody would mind a better OS, but when the GUI has reached the pinnacle of usefulness, why try to force a change?
Your entire point revolves around the idea that the UI has reached the pinnacle of usefulness. I disagree, but I understand why you may not. I assume you use your computer sitting down with a mouse and keyboard?
I use mine like that as well. I also fold my keyboard away and then touch the screen. Sometimes I write on it with a pen. Windows 7 was unusable in this scenario. Windows 8 was woeful. Windows 8.1 was a significant improvement.
Your car analogy assumes that the user will keep using the car the same way and that the vendor is changing the rules. That is not the case. How many motorcycles have you seen with a steering wheel and 3 pedals for control?
The idea of the computer is changing. That change is largely driven by touch screens. For touch screens the UI needs to change. A lot of elements of windows 7 make the UI difficult to use including small touch targets, no gestures, no ability to use it without a keyboard, the inability to detect if the screen was being touched with a tip of a pen or the hand holding the pen etc.
Have we reached some pinnacle now? Heck no, it's a major work in progress. But not everything is a case of change for change sake. This is Microsoft responding to users replacing PCs with tablets, not Microsoft driving (because lets face it they are pretty damn bad at that too).