"even the chipset doesn't know how power rails are wired up"

Hmmmm looks like ACPI is required because the hardware design is stupid.
The motherboard chipset should should have specification of exactly how it is wired up to the ATX PSU etc.
I can't see any reason why this isn't standardised across all motherboard chipsets.
Hardware manufacturers have done a bad job and we are stuck wtih ACPI as bandaid solution it seems.