(mainly for above-bookcase/furniture lighting)
So no where nearly as obvious as mine that are just above the mantelpiece and hence easier for visitors to see.
They are still readily available
e.g. click here
Oh sure, but I would have thought not as commonly used as when mine were put in (c. 1934). When XYZ technology stops frequently being installed or attached to devices by default that does not mean that
everyone stops making, selling & using it. At times, like your example, for different uses than originally intended and at times for legacy applications.
They were not installed in the millions even in the 1930s but I am sure the usage now is a lot less.
True, but their widespread use for charging mobile devices is appreciably more recent that that.
Certainly more recent than mid 90s but I'm not sure I'd say
appreciably more recent. USB has always had power as well as data and before devices used proper (i.e. standards compliant) USB connections, many manufactures used USB-type cables to connect & power their devices but with software on the computer (or a dedicated wall-wart) so you had to buy a proprietary product. Those normally used mini-B sockets on the device and mini-B was in 2.0 standard which came out in 2000. So I suspect these were common by the mid 2000s, they certainly were common by 2009, I had a mobile phone like that then and there were masses of USB-type proprietary chargers.
I may, of course, be wrong, but I do suspect that it will not be all that long before we (generally) use some different method of charging mobile devices.
No real idea. USB will still be common, as you said, for at least a decade. If something clearly better comes along then it drop back reasonably quickly on new devices. However it is being updated (for data and for power) so anything that replaces it will need to have a clear advantage to make a change worthwhile.