There would be no choice, there would be no 5gbps USB mode so there's nothing you can do but use a PCIe chip. It would've brought down the PCIe costs over the many years.
Ok, yeah, I agree. But the world of cost-cutting and penny-saving would never allow that - same reason FireWire lost out to USB. As passive and dumb peripherals as possible won out (for cheaper parts and faster time to market).