Compare that to a typical switch-mode power supply, which is already very efficient. The quiescent current of a modern, reasonably well designed power supply is basically negligible. The problems of 'vampire draw' have basically been solved for the actual power supply (devices that don't 'sleep' are still an issue, but that's no fault of the supply).
Recently I modified some LED fairy lights with a LDR/transistor setup. The quiescent draw during the day is about a tenth of a watt, vs. 6 watts under load. Left plugged in for a year in 'off' mode, that would cost me less than 10 cents.
This 'variable clock' switch-mode supply is all about squeezing every bit of battery life you can with a device. It's about making sensors that can be deployed without a power source that last for years instead of months.
Long-distance power conductors use the highest practical voltage to avoid what are called "I squared R" losses, so named because the power dissipated in a resistance (like a transmission line) is equal the the square of the current times the resistance.
For a given power level, one can obtain that same power with a very high voltage and a proportionally lower current, thus greatly reducing resistive losses in the lines. This is why a million volts is by no means out of the question for long power transmission lines.
It used to be much easier to step the voltage up with AC, but now we have advanced enough power electronics to make the inherent increased efficiency of HVDC be actually worth it.
This question seems to come up every time there's a discussion like this about low-voltage power. The whole reason we use high voltages in the first place is to reduce resistive losses. Ohm's Law (V=IR) plus the formula for power(P=VI) tell you everything you need to know, which is that power is related to current and resistance by the equation P=R*I^2. So for a given amount of power, if you increase the voltage by a factor of 2, you then decrease the current by a factor of 2, and you reduce the resistance by a factor of 4, so the power lost in transmission is 1/4 as much.
If you tried putting 5V USB ports in your house, you wouldn't be able to use wire to connect them to the central power supply; you'd need to use massively-thick copper bus bars. The cost would be astronomical, and you'd need huge channels on the walls to hold them.
Any more current than 1-2A the wiring gets silly. At 5A, which is around the power levels used for LED lighting, you need 6AWG (13 mm^2) wires.
These figures were calculated assuming an average of 40' of wire from converter to outlet. A per-room converter would be more reasonable, but you might be better off doing something like using 120vDC so every non-motor device can drop the bridge rectifier.
But this article is mainly helpful for battery and mobile usage of IoT devices. At home, the biggest energy guzzlers are the components of your HVAC system, heating, air-conditioning, refrigeration, electric stove, etc. Their power consumption scale is roughly a million times that of all the IoT devices combined. You're not saving anything if you're only making IoT power consumption more efficient at home.
What? PoE (802.3af/at) operates at 48V DC. [0]
There is no AC being transmitted in PoE. 48V DC is used to avoid power loss due to small gauge wires in CAT5/6 cables, so they can provide power to devices up 100m as per the Ethernet specification.