Case in point: the stairwell shin-height lights in this 90s house are 12 VDC. There's a transformer plugged into a wall outlet in the nearby storage closet.

That works OK because the transformer is relatively close to the lights. If it were a reasonably-large house, and the transformer were on the opposite side of the house, you'd have a problem with a noticeable voltage drop. All these ideas people are throwing out here involve a single whole-house power supply. If it were for 48VDC, it would probably be fine, but 12V would result in significant line losses.

We already see transformers for a run of e.g. track lights, low voltage lights on tension wires, and so on. That's been a thing ever since halogens came to market.

Having multiple transformers is perfectly doable and commercially viable -- though I would appreciate more product availability for something easy to stash in the hollow space of a ceiling, like recessed lighting is installed.

I still don't see the point of all this. If you have a handful of lights in a room, and drive them with a single power supply, you're still going to have big problems: the line lengths to each fixture will be different, resulting in different voltages. You can't drive LEDs that way with good results: they need fixed current. And you can't daisy-chain them either: if one emitter dies, then the remaining ones will suddenly have different current, and probably die quickly. The proper way to drive LEDs is with a power supply very close to the emitters and designed specifically for those emitters and the (short) wire length to them, not 4 meters away and not with some variable-length wire that can't be designed for.

Everyone here is complaining about ultra-cheap LEDs that don't last very long because they're poorly engineered, but that's exactly what you're all trying to do here by using a separate, shared power supply. You could get away with that in the 1980s using incandescent bulbs, but you can't do it now unless you want the same crappy lifespan and reliability you're all complaining about.

The solution is very simple: buy fixtures that are engineered well. Switch-mode power supply electronics are not expensive at all, but when mfgs cheap out or do a crappy job designing them, you get bad results, usually short lifetime of either the power supply or the LED. What you're trying to do here is buy a really expensive power supply, which has to be engineered to a far greater degree and for a far wider range of operating conditions (since they don't know what you're going to connect to it), just because you had a bad experience buying some $2 light bulb that had a crappy power supply built-in. This really makes no sense.

12V requires quite a lot of amps for enough light, so low DC is not optimal. Also LEDs are current driven devices, i.e. they will be sensitive to voltage changes (even with a current limiting resistor)

Low-voltage doesn't necessarily mean 12V. I think it's anything below about 50, although lighting systems currently marketed as "low voltage" are usually 12 or 24 volts.

The constant current thing is true, but that's not a terribly difficult problem.

It's complicated. https://en.wikipedia.org/wiki/Low_voltage

Depending on who you ask, the limit for low voltage DC might be 42 or 50 or maybe 60 or 120 or 1500.