Or cellular phones, computers, refrigeration, microwaves, electricity, AC and a thousand other things that only rich people used to be able to afford.

Just about any luxury of the rich that can be produced more efficiently over time makes its way to the average consumer.

Out of the loop here, what were the big issues with early 4K TVs?

4K resolution, but only support for HDMI input at 30hz, or having to use some incredibly fiddly solutions where two cables had to be used at once. Internal image processing was too slow, so you would see 100ms delay which made them too slow for playing games. And then the quality of 4K panels was frequently far below good 1080p, but for more money.