No, they don't, not at all.

Most modern appliances have variable-speed motors these days. You can't do that by just connecting AC to a motor; you need a control board to generate the waveforms necessary to make the motor turn at the speed and direction you want. That control board has to be fed with DC power. (Source: I used to design BLDC motor control systems)

Only really simple appliances, like old-fashioned horribly-inefficient clothes dryers, still use AC induction motors, and those are mostly being phased out. (Bathroom fans also need AC; they're usually cheap synchronous reluctance motors.)

So it really doesn't matter much whether the incoming power is AC or DC these days, unless you have a bunch of ancient appliances that still use induction motors. If it's AC, it's going to be rectified and fed into a DC-to-DC converter to create the lower DC voltages needed. If it's a higher DC voltage, we can skip the rectification step and not worry much about ripple.

Indeed. And that’s quite normal. Our electrical system should serve our modern needs.

> but you can't run that through a home

5V might be too low for that length of wire. But you could most definitely have a low voltage line in your house that we could design around, maybe 12V. Electric vehicles are moving towards 48V for accessories. It seems like lack of a standard is holding us back more than anything else.

Or we could just keep doing 120V in the walls, with a DC supply. Modern DC-to-DC voltage converters are very efficient and small. But maybe I’m wrong. A lot of people seem to believe they are still not good enough yet for such a change to make sense.

> If you're going to have AC and DC then you might as well just have AC.

I arrive at the opposite conclusion. Most things are natively DC. So therefore, power in the walls should be DC and we should covert it to AC near the endpoint where necessary.