Most of the motherboard's power consumption is dedicated to supplying the CPU, and that already relies on the motherboard to step down from 12V to ~1V at a very

Most of the motherboard's power consumption is dedicated to supplying the CPU, and that already relies on the motherboard to step down from 12V to ~1V at a very high current. The demands on the 3.3V and 5V rails are pretty small in comparison. This change is unlikely to make motherboards significantly more failure-prone than they are now.