The issue is that the lane is physically dedicated to the USB controller. It’s not like all USB devices share a common pool of lanes. A USB port will connect to a USB controller, which is connected to a lane. If you have USB 2.0 controllers, as mentioned before you can have about 10 using a single lane and be fine, but for a 3.0 or 3.1 port the lane will be assigned to it and can’t be used for something else like an SSD or GPU, even if you only use it for a 2.0 device. Since we can expect many devices (eg mice, keyboards, webcams, printers, etc) still oy need 2.0, we don’t want to waste bandwidth and so we’ll have 2.0 ports, with a few 3.x ones for storage devices, high speed network adapters, external GPUs, etc.