I could see AMD possibly implementing PCIe 6.0 on the CPU, but let mobo makers downgrade it to PCIe 5.0 or 4.0 in the consumer space on X and B series motherboards. Even split it further to 3.0 on A series motherboards. Something roughly like: (I know it's likely not correct, but trying to get the idea across)
- X-series Motherboards see the total x20 or x24 6.0 lanes converted to a total of x40/x48 5.0 lanes or even x80/x96 4.0 lanes or some combination thereof of 4.0 and 5.0 lanes.
- B-series Motherboards see the 6.0 lanes converted to x80/x96 4.0 lanes
- A-series Motherboards see the 6.0 lanes converted to 1 x16 PCIe 4.0 GPU Slot and 1 x4 NVMe Slot, and the remaining 6.0 lanes converted to x120/x152 3.0 lanes.
With PCIe 4.0 and 5.0 needing redrivers/signal repeaters, it'll take a long while of usage in the consumer space before those ever become cheap enough, or if there's a material breakthrough in room temperature superconductors that does away with them (or a complete design shift to the next motherboard and PSU standard). So aside from halo X-series Motherboards pushing 5.0, cheaper ones might only use 5.0 for the 1st GPU and 1st NVMe drive, and the rest using 4.0, which is good enough for future-proofing for awhile, considering that only storage and Ethernet have immediate benefits from more bandwidth (assuming one has the appropriate add-in cards).
PCIe 6.0 will really only be useful for cloud datacenters and research institutions that need more than 500 GbE networking, as beyond that there isn't yet any sort of non-proprietary hardware that could really utilize that amount of bandwidth, especially not at the consumer level. Further, it might be preferable to use cables for PCIe 5.0 and 6.0, considering that they're cheaper than relying on retimers/redrivers and specialized PCBs. Kind of like the PCIe version of the classic SATA cable, but now plugging into add-in cards alongside the power cables (or maybe even replacing the power cables; should the shift to pure 12v PSUs and motherboards mean redesigning power delivery as a whole).