I don't know the implementation details of this, but I use both a 3950x and a 2080 Ti for rendering in Blender's Cycles, which is a non-realtime path-traced engine. I've experimented with several different configurations, and the GPU using its raytracing cores is much faster than the CPU for identical results. Given that next-gen cards are expected to increase the number of these cores by 2x or 4x, I don't see how software raytracing could compete.