The gap in graphical fidelity is so monumental
I'm sorry, but where exactly do you see this monumental gap, because it simply is not there. Rather than making overblown general statements, let me comment on each of the images:
First comparison: Completely different lighting and camera angle, VGX image is oversharpened to ludicrous levels (just look at the artefacts on the grass in front of the mill,) the actual assets are identical. In motipn, it has some nice wind effects, which we've seen in other areas of the latest gameplay footage.
Second image. Again, different colour saturation (on the flame, I imagine it would look exactly the same if we had this same scene on the latest build that was shown, which has a much higher saturation level than the 35-min demo did) and a brighter night (it's a full moon). The assets are of the same quality.
Third comparison: Comparing screenshots taken under perfect conditions to gameplay stills. Okay... Overlooking the impeccable composition of the shots in these handpicked screenshots, the assets are still the same. The griffin and Geralt in the screenshot of the supposed pre-downgrade version also seem to have a wet effect on them, which is weird.
Swamp comparison: Assets are, again, literally the same (look at the trees.) Different lighting setting, less saturated colours on the 35-minute demo. Also, VGX footage has more bloom (blinding reflection from Geralt's armour) and sharpening (grass looks like it's going to cut you.)
Geralt's face - Yeah, that scar looked pretty low-res and lacked depth in the SoD trailer shot, didn't it. Beard looks a bit shit, too. And will you look at that non-nVidia hair. Ugh.
Siren looking funny - I would cherry-pick an image of a siren from the Debut trailer if my laptop would allow it.
Geralt without shading - Uhm, I don't get it. It shows Geralt's face without shader effects.
Gifs - yeah, those are gifs of SoD alright. By the way, the purple haze in the horse scene was actually mentioned in one of the previews, guy said it looked amazing, if a bit funny when the light was reflected on character models in cutscenes. The water effects during a storm look amazing, too, hope we get to see more of those as we get closer to launch.
Bottomline: I don't see this "obvious" downgrade. It's just the nature of an open world game to not look consistent all the time.