And as soon as you have a failure, the entire house goes dark
Luminary could include an idling diac (..)
What on earth would be the advantage of that??
I can see various advantages, including lowering investment cost, energy bills and also maintenance.
The wiring of luminaries stays the same for series and parallel connection, just the arrangement changes but cables, voltages, cross sections, circuit breakers and switches stay about the same. If something was designed for 230VAC, it won't work at 1kV reliably. Also, if you design a 1A DC luminary, it is not going to work reliably when fed at 5A - that is quite understandable, you'll get a fire.
As for energy - creating a 5W current source inside of a tiny bulb crammed into luminary has serious limitations as then heat comes from LEDs and the source in a small envelope and that raises the investment (mainly BOM) and affects reliability. When a physically big and efficient source can be installed in a DIN rail inside of an electrical cabinet, such constraints disappear. As for dimming - of course there is no problem in installing three variable current sources per one toilet - that is not particularly complicated but won't work with incandescent bulb triac dimmers (obviously). The question is do you need so many dimmers, maybe one dimmer per room is enough? Also, the luminary itself can include a series / parallel local switch or some DC/DC for more demanding customers.
So, I cannot see any technological limitations to make future residential lighting to be powered from current sources. What is more, I believe that once people stop converting kerosene lamps to incandescent (or was it: converting incandescent to LED actually) then in 10 or maybe 20 years, after dinosaurs get extinct, a current source is what is going to be a standard for general lighting.