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.