> One reason DC power is more viable nowadays is (as others here have mentioned) the use of DC-DC converters. They internally actually use AC to do the conversion (usually at much higher frequency to save weight and cost on inductive elements). But they're not super cheap (not cheaper than transformers) and they can also be inefficient.
DC-DC converters are cheaper than the classic transformers. There's a reason that you almost never see the classic "big chunk of iron" wall-warts anymore: not only are they inefficient, they're more expensive.
> Low voltage DC (like 12V) requires MUCH thicker cables for the same power, which means a lot more copper (and copper mining). Typical line voltage can be very lightweight.
Yes, a low voltage DC bus doesn't make much sense except for specific applications. Even in small spaces like recreational boats it's more common to use 24V or 48V these days. I think around 50V makes sense, as it's not very dangerous so there are fewer safety regulations. Look at PoE, for example. Unprotected runs of fairly thin cable transmitting up to 52V.
> With DC, nothing is really at the same voltage, so you need DC-DC converters all over anyway, so you're not saving anything (although not losing much, either!).
DC-DC buck converters are the simplest, cheapest, and most efficient category of power converters at the moment. (Of course a specific converter may be worse than a specific converter of a different category, but this is broadly true) Buck converters that support up to 50V are only marginally more expensive than lower-voltage ones. As an electronics engineer, I would be very happy if I could get away with only needing buck converters from now on; they're very easy.
If the DC source is required to be galvanically isolated and to have it's own transient suppression than the DC devices can get away with much less input protection than AC devices require. Could be a significant cost savings.
> Another important thing that really drives some of the advantages is: breakers, relays, and (much less important) current measurements of existing cables. [...]
Absolutely. I don't see high voltage/high power DC becoming popular in homes any time soon for that reason. There shouldn't be any difficulty with arcing at 50V though. (Yes, switch contacts still arc but it's not any more difficult than 120VAC) Also high power applications in houses tend to work just fine on AC (motors and restive heaters), though that may be a chicken-and-egg thing.