I have read a popular post before from an "electrical engineer" making the same claims as you. The flaw is that most risers still draw power from the MB, if they didn't then you would be correct. Have you ever tested this out? If OP had the PSU powering the MB connected to all the risers, and the MB molex coming from the same PSU that powered the MB 24pin/12v. then he never would have ran into a issue.
I don't think MB manufacturers have got this wrong, I think you've got it wrong. Some boards will not post with more than 3-4x GPUs without a molex plugged in. It's only a problem if the molex you are using is coming from the PSU that's not plugged into the motherboards 24pin/12v connector.
I've said it before and I'll say it again, you do NOT need to use the Molex connector on any motherboard when powered risers are used. Doing so is dangerous as seen in this case. Built in safety nets are bypassed when using that out-of-spec setup. There is very little power draw through the USB riser cable. It's primary function is to pass data, not power. If there is more than a couple watts passing through, then the powered riser is not doing it's job and is junk. The molex connector design on MB's was only put in place to facilitate multi GPU's directly connected in the slots for Crossfire/SLI configurations on Gaming MB's that had two or more cards running in the full length slots. It is an unnecessary design to have it on a mining board with 1x slot when powered risers are used. Molex connectors weren't even designed to exceed 36 watts so it's pointless to even have one molex boost 12 PCIE slots, it would theoretically only supply 3 watts per slot anyway. It is a design flaw that the Chinese just winged at and completely ignored the power limits and design standards of the components used. This fire hazard could have been avoided if the molex connector simply had not been used. The MB would have likely just shut down in the OP's situation instead of running current through an unmonitored bus. I would be more worried about having a separate switching PSU power the GPU PCIE 12V bus than the 6/8 pin 12V PSU. There's two high current differentials waiting to create a disaster right within the GPU itself.
Some motherboards will not post when more than 3 or 4 cards are plugged into, without a molex connector plugged into the MB, it's programmed into the bios. eg most x99 boards are like this. What are you suppose to do then? tbh I don't really care about molex and mbs. I'm only interested in dual psu configurations
I have a multiple rigs setup that use 1x PSU for MB + risers, and the 1xPSU to power the 6/8pin, they have been running for over 6 months . I originally read that post linked below from mid 2017 by the "electrical engineer" and set my first rig up so that each riser + card was powered by one PSU and the other PSU was used to only power the Motherboard. After turning on the PSU that was powering the riser/ cards, and turning on the PSU that was powering the MB, my motherboard started smoking. I thought i'd fried it!
When I started reading other posts about dual PSU's problems, all the problems came from people who were mixing and matching PSUs, and had a combination of different PSUs powering the risers and MB. I then came across a video from the pop Youtuber- "Bits Be Trippin" who said to use the same PSU for MB & risers, and then the slave PSU for the 6/8pin, otherwise you run the risk of creating ground loops. That dude would have built 100s of dual PSU rigs. How many have you built? After following this advice I've never had any issues running dual PSUs( I have 8x rigs ranging from 7 to 12 cards each)
You said above that "There is very little power draw through the USB riser cable". Even if there is very little power draw, this is still power draw. So the assumption that there is no power connection between the riser and motherboard is false.Please think about this some more.
