What's wrong with PCIe, and who is going to trust the longevity of a motherboard manufacturer's proprietary interface?If Intel, at some point in the future, truly ditches socketed processors for the consumer market and moves to BGA-only bulk shipments, that doesn't mean there shouldn't be some proprietary interfacing standard to replace it.
I just pointed out with interposer or on-package DRAM that there's not even the need for a tiny PCB.but to no avail. Now with the advancement of the lithography and integration one could have a powerful CPU+IGP and multi-gigabyte of fast memory (high-density DDR4) soldered on a single small PCB. The main board would still handle most of the periphery and maybe direct integration of SSD storage in some form.
Intel also intends to put the southbridge on-package, so the periphery is basically whatever wires are needed to go from CPU package to some I/O hanging off PCIe.
SSD interfaces are already bottlenecking higher-end drives, and it seems the future direction is SSD to PCIe or some variation thereof.