4 3.0 lanes may be enough for a GPU, but if you're using a 3.0 capable GPU on a 2.0 system, you'll need 8 lanes. So my guess for why GPUs have 16 lane connectors is for backwards compatibility. What motherboard manufactures could do is use open ended x4 slots. But from what I've observed, people don't seem to know that you can put an x1 card into an x4 slot
And the future is actually less bandwidth requirement:
> Performance doesn't even drop with newer DirectX 12 and Vulkan games, including titles like "DOOM," which are known to utilize virtual texturing ("mega textures," an API feature analogous to Direct3D tiled-resources). If anything, mega textures has reduced the GPU's bandwidth load on the PCI-Express bus.
So: 3.0 x4 will be plenty for the foreseeable future.
