May be somebody gotta implement a different rendering for the main picture and for the rays of reflections and shadows/shades/ambient occlusion. Do the picture in 4k, then apply to it rays is 2k. Reflections and shadows do not have to be of the same sharpness. Just look at the reflections and shadows around you, they never sharp unless its mirrors (and well polished ones) or a hard artificial lighting. Reflections in puddles and wet dirt after rain also do not look like made of nvidia mercury with lacquer sprayed thereon.
If somebody gotta implement it, I guess it will be AMD (call it Radeon Rays 2.0 or something). And they can utilize unused cores from ryzen cpus, e.g. a combination of 16-cores threadripper, which is $900, and vega "128", which hopefully be around $599. AMD did the mantle renderer which entailed dx12 and vulkan. What Nvidia did? An expensive video card. AMD did freesync which is already even a part of the hdmi 2.1 standard. What Nvidia did? An expensive monitor expansion board (gsync). Well, AMD did competitive 8/16/32/64 cores cpus. What Nvidia did? A Nintendo Switch SoC. Alright, that one wasn't too bad.