Actually, whenever Microsoft pushes out a major change to their Operating Systems (OS) there are quite a few bugs. With the hundreds of thousands of devices that the OS is used on--PCs, laptops, tablets, phones, et al--I was expecting there to be a lot...but so was Microsoft. There was no feasible way for them to truly identify every single piece of hardware and software, and ensure 100% compatibility and compliance!
The problem with 'rolling back to 8.1' is that I didn't get a copy of the software when I bought the tablet, and I don't have certificate or a "serial number" indicating that I have a legitimate copy of that version of Windows. I am sure that if I go through Hewlett Packard I might be able to get the older version.
And you are right, you can replace ANY version of the software with something else (as long as the hardware is compatible). There is a plethora of operating systems out there: Windows, Linux, Unix, and others. Windows is just the accepted norm for the most part.