From what I understand, listening to talks about the development of DirectX12, it was made with backwards compatibility in mind and to be somewhat future-proof, for posterity, with "new shinies" always able to fall back to less demanding (thus more compatible) alternative rendering techniques. So, it is very true that it can be scaled because the programmers themselves said it. You wouldn't be arguing with just me about it, although, this was a number of years ago now. The issue then is not the API itself or the programmers working on the technology itself, but how such compatibility may conflict with certain commercial interests, if it means less people throwing cash at new hardware. I'm convinced of that. Is that Halo 2 would ONLY work on Vista for purely hardware reasons, was it "we just HAD to do that", or was it to just sell more copies of Vista? How did that turn out?
I personally am not interested in constantly throwing out all of my hardware and buying everything new, such so the new eyelash-physics and raytraced reflections in the forehead-sweat will work, or the game will refuse to work at all. At this point, I'm convinced it's being done on purpose, simply because it conflicts with executive interests and their revenue streams, and sure, also saves money on paying programmers extra to make the extra tweaks to make everything more compatible.