I should also point out that ray traversal / intersection costs are generally only around 40% of the costs of extremely large scenes, and that's predominantly w

I should also point out that ray traversal / intersection costs are generally only around 40% of the costs of extremely large scenes, and that's predominantly where GPUs are currently much faster than CPUs. (I'm aware of the OSL batching/GPU work that's taking place, but it remains to be seen how well that's going to work). From what I've heard from friends in the industry (at other companies) who are using GPU versions of Arnold, the numbers are no-where near as good as the upper numbers you're claiming when rendering at final fidelity (i.e. with AOVs and Deep output), so again, the use-cases - at least for high-end VFX with GPU - are still mostly for lookdev and lighting blocking iterative workflow from what I understand. Which is still an advantage and provides clear benefits in terms of iteration time over CPU renderers, but it's not a complete win, and so far, only the smaller studios have started dipping their toes in the water. Also, the advent of AMD Epyc has finally thrown some competitiveness back to CPU rendering, so it's now possible to get a machine with x2 as many cores for close to half the price, which has given CPU rendering a further shot in the arm.