Recent versions of PCIe have short reach within a system precisely because the physical layer is not all that tightly controlled. Cheap PCB materials are a much

Recent versions of PCIe have short reach within a system precisely because the physical layer is not all that tightly controlled. Cheap PCB materials are a much worse physical medium for high speed signals than twinaxial cables, which are used in a lot of rackmount servers to bring PCIe from the back of the system where the CPUs and expansion slots are up to a SSD backplane in the front of the system. Thunderbolt goes for even longer cable runs by using active cables, while long PCIe runs use redrivers or retimers that aren't integrated into the cables. Thunderbolt is also not really a downrated PCIe in any way; Thunderbolt transmits data faster than PCIe 4.0 lanes, but doesn't scale to as many lanes as internal PCIe links.