The Witcher 3's biggest technical challenge is the asset density and complexity combined with the total seamless data streaming, as in being able to travel from the far south of Velen to the upper north of Novigrad, going through caves, forests, swamps, houses, castles, towns, cities, ruins, etc without being confronted with a single loading screen. It's the biggest achievement of the engine, to cover such a large variety of topographical variances while also rendering interiors and fluctuating NPC and geometric density, while continuously adapting to cycles of time of day, weather, and NPC behaviours.
I don't think it's impossible mind you, because discussing any port of any game to any hardware configuration comes with dependency on target (and fluctuating) framerate alongside asset and shader quality standards, factors of which we know nothing about (let alone know if this is even true). Scale back parts of The Witcher 3 far enough and it'll probably work. The real question is how far said scaling will go (draw distance of foliage, NPC density in cities, lighting/shadow quality, etc) and what kind of performance baseline CDPR care for.
If it exists it's obviously not going to look as good as the PS4/XONE build, and certainly not even in the ballpark of the PC build. But still, if they can get it working and looking servicable then more power to them.
