Arguably, it's now cheaper in raw material terms to up-step and down step DC with high frequency transformers and buck-boost. The problems are lower efficiency than a modern, high efficiency transformer, which leads to increased dissipation and lower life of the complex electronics involved, requiring active cooling. An AC transformer is a well understood technology that can operate for decades without any maintenance. DC-DC high power convertors are exotic beasts with large downtimes and expensive to repair and maintain.

We are a long way of seeing this setup used outside very special applications, AC might no longer have a natural advantage but the benefits of DC are too small to justify the massive investments and research required.

Though, it can certainly be argued that a single home-wide rectifier and smart DC wall sockets could improve efficiency and safety. You could have things like negotiated power up, ground insulation, cheaper and more efficient light bulbs and appliances etc.

People are already building 802.3af and 802.3ad based office LED lighting systems at 48VDC, since it's much cheaper to run cat5e around a big building.

Don't forget that three phase allows for three conductors to carry the power of three pairs of single phase conductors.

Three-phase AC also allows for very electromechanically simple generators and motors. Industrial electrical loads have traditionally been (and often still are) rotating machinery, putting three-phase at an advantage.

That's not entirely true. Any load imbalance on the phases is resolved through the null, connected to earth at both ends of the line. You can do the same with three single phase lines, although you wound need a massive earthing system with low resistance.

Or you could build a 2 wire (or 4, 6 ...) DC line with complementary polarity that would carry more power due to skin effects. If the negative and positive loads are balanced, the currents though the earthing wire cancel out.

Alternatively, skin effect limits the amount of AC current you can throw over a single conductor. With DC, you can make your conductors arbitrarily thick and you'll use the whole thing.