Not like for like comparision. You are comparing two operations systems, yes. However, the business models are completely different. Do no forget that Apple is a hardware company that makes software. Microsoft is a software company that makes a bit of hardware. See the difference? Windows needs to cater to every single machine it can, because the OS is the product. Same with Linux, UNIX, etc. MacOS is not the product in and of itself. It is a integral part of the product package, that only runs on Apple hardware. Same with iOS and iPad/iPhone.
Of course sales politics has to do with it. Planned obsolescence naive to believe otherwise. As illustrated, the hardware is Apple's business model, not the software. Then again, 8 years is a good run for any computer, Mac, Wintel, whatever, and you already know Apple's stance on computers that are more than 5 years old. Owning one is "sad" according to Phil Schiller, and it quite makes sense that he says so, right?
If you want to keep using your older Mac Pro on the very latest operating system, maybe you have to go to Windows 10, but I would argue that, "What's wrong with El Capitan that you can't just keep using that? Why is that not an option for you?"
I get that you want to have the latest and greatest features on your computer, but taking things in perspective...have you tried running Windows 10 on an average, run of the mill 2009 Wintel machine? It isn't very pleasant, I can tell you that!