PC ATX12VO (12V only) standard - Why does everybody say it has higher efficiency?
Modern PC parts still need those rails, for example, SSDs usually use 5V for main operation.

No, NVMe SSDs as attached by M.2 get a 1.5 V and a 3.3 V rail,

We use 5V in USB devices to charge up cellphones and power personal beverage coolers. So the need is not going away (any time soon) just because Intel has published a new standard.

USB is a very good example of something that will not change, indeed.

Why does moving the 5V and 3.3V rails to a different place ...
Why not throwing a few more components into the ATX power supply &quot;box&quot; and down-convert from the high-demand 12V rails?

Because: you can't drive anything that draws any significant current at 5V if that comes from your ATX supply some 40cm of cable away – voltage drop in the cable. You can't drive any high-speed circuitry with a constant voltage through cabling, either – parasitic inductivity will mean you simply can't react to the quickly changing current draw.
So, you always need to convert close to your large loads. That's why there's arrays of rather beefy DC/DC converters around CPU sockets!
With the converters close to the load, you can build many converters that are optimized for the load profile of their individual load – and hence, more efficient on average – than one necessarily massively overdimensioned converter, which nearly never runs close to maximum efficiency.

Linear regulators commonly found on cheap adapters like M.2 to SATA generates 3.3V rails by consuming all the power from the rest of 8.7V as a space heater. Aren't these more problematic and should be eliminated first?

Yes, but these things are aftermarket, and I've not seen anyone use one of these in a modern system. They are effectively never a problem. &quot;My car users sometimes throw an anchor out their window, isn't that more problematic than another liter per 100 km off for a car?&quot; comes to mind....

But you can easily design a multi-stage converter using the main just once.

You seem to be quite the expert on converter design, then... I'd see quite a few problem building that to a high degree of efficiency.

Also, don't underestimate the idle current savings you get when you get rid of a bunch of rails that your target system probably doesn't even need.