From what I gathered from an AI conversation, there might actually be a good reason to skip USB 2.0 and go straight to USB 3.0 for older motherboards.
The reason for that is that the xHCI controller in USB 3.0 cards is more intelligent than the EHCI controller in USB 2.0 cards and effectively offloads the CPU and does a lot more of the processing independently from the CPU.
That is the case, regardless of the generation of the USB device that you plug into such a card (USB 1.0, USB 1.1, USB 2.0, USB 3.0). All such devices are handled by the xHCI controller. This is different from USB 2.0, where the card would have an EHCI controller just for USB 2.0 devices, plus a OHCI or UHCI controller for USB 1.x devices.
There are USB 3.0 cards for conventional PCI slots, although those are always built around a PCIe-to-PCI bridge chip, since no USB 3.0 (xHCI) controllers were ever designed with integrated conventional PCI interfaces.
Because of the improved way how xHCI works, it shouldn't degrade the performance of older host systems, at least not as much as USB 2.0 (EHCI) cards do, even if you'll never be able to utilize the maximum bandwidth of USB 3.0, due to the limitations of the conventional PCI bus.
Of course this is theoretical. I have purchased such a conventional PCI card with a USB 3.0 (xHCI) controller and intend to test it on an older system soon.