Isn’t this the idea of having a chipset (Northbridge/Southbridge) on the board, to handle some of these IO tasks?
Well, not in the way we are talking here. The bridges work under the CPU’s direct commands. When the CPU goes to sleep, they have nothing to do either.
Maybe a PC’s CPU can do some short time limited sleep like microcontrollers do, and maybe that would be useful for such scenario’s. But even then the main problem remains that the CPU wants to be in control of everything.