There is nothing stopping anyone making a motherboard that has no pci or pci express. All pci cards can be replaced by sata, usb2/3, firewire or optical fiber. Personally all devices should just use optical as this is what we are going to use in the future anyway. The only device that would cause problems would be the graphics card. It should be possible to replace pci express with a optical connector as optical fiber can handle 1Tbps and above. With no pci the motherboard can be cut in half or to a third of its size thus making it cheaper.
My ideal pc would be a computer of just 3.5 inch double height and single height bays. Motherboard would take 1 double height bay with its own internal fan and heat sink, you could add another motherboard and link it to the other one using optical fiber. My graphics card would take 1 , so for a sli or more I would just buy 1 or more cards and plug them into there own bays and connect using optical fiber and that’s it.