Why is it impossible to build a new Windows 10? Shouldn't that be an area of great interest to Microsoft? What other endeavor would give Microsoft as great a leap forward as a ground up rebuild of the operating system?
It's not impossible, but it will take an enormous amount of effort to replicate. Consider the ReactOS project, which strives for compatibility with Windows Server 2003. I use it occasionally in a VM and it is roughly at par with Windows 2000/XP, but this project has been going on for over two decades (granted, the target has moved over the years; originally it targeted Windows NT 4.0 if I remember correctly). It will take years of work for ReactOS to reach compatibility with Windows 10.