But doubling the bandwidth per lane allows one to use half as many lanes to a GPU and maintain bandwidth. As you mention, it allows an eight lane GPU to be a viable option. And better yet, due to how PCIe handles variable number of lanes between host and device, different users with the same CPU, GPU, and even motherboard can choose to run the GPU at eight lane with a couple of four-lane SSDs, or at sixteen lane for even more bandwidth if they don't need that bandwidth elsewhere.
8 lane GPU is viable for a long time (benchmarks on PCIe 8x versus 16x shows a 5% perf difference), but it did not change the physical layout the motherboard manufacturers use; you cannot use the existing lanes any way you want, on some motherboards you cannot even split it the way you want between physical connectors and video card manufacturers continue to push 16x everywhere.
