I expect motherboards could be more reliable if there was a standard 12v to 5v, 3.3v, 1.5v etc pluggable converter that they all used. It could cut down on E waste. Instead of having to discard an entire motherboard, the low voltage converter board could be swapped out and repaired/discarded. The same companies that make ATX power supplies could also make the discreet low voltage converters. ATX innards could be smaller, less wiring between components.

No that's silly, we should continue to use a design from 1995, itself replacing a design from 1981.