My point was that the economics of higher-volume manufacturing result in eventually subtracting everything to the point that the "backplane" is the motherboard.

My point was that the economics of higher-volume manufacturing result in eventually subtracting everything to the point that the "backplane" is the motherboard. Early ISA backplanes were passive and you basically just had a PC on an ISA card plugged into the ISA bus, with maybe some extra connectors for power and peripherals. Then PCI backplanes started out this way but quickly gathered enhancements that resulted in the "System Host Board" interface and a special connector for more power and IO to/from the "CPU card" in order to do things like support ISA, PCI, peripherals, and later PCI-X slots. Rather than having the extra complexity of the connectors, the high volume PC market folded all of this "onto the backplane" creating the modern motherboard. It might not be clear unless you know a bit about early computer backplanes like S-50/S-100, STD bus, STEbus, VME bus and the equivalent Passive ISA backplanes... by the 90s this is well underway and we already have motherboards that you could recognise as "a motherboard" and "backplanes" that are noticeably different, having evolutionarily diverged during the 80s under differing economic pressures.