Re: "a decent fibre network connection (around 15Mbps)"
More than half of them get less than average?
Actually it's very complicated as a few 100Mbps to 1Gbps can mean even though half of people get less than 5Mbps an "average" can be 15Mbps.
Packages on copper are misleading. Up to 22Mbps can be average of less than 3Mbps due to crosstalk, line quality and distance.
There are different kinds of "averages". The median user speed at peak time is more meaningful if the SD is also quoted. A graph with two lines is best. (% users vs speed buckets for peak time and off peak)
Also what if two or more people at a premises want different content?
UHD or even HD, never mind 2K on internet in reality needs a video server at each ISP node and fibre to the home otherwise only a tiny handful can watch it.
We don't even get the quality possible on SD or HD, because of Terrestrial spectrum constraints (too much going to Mobile) and cost on Cable and Satellite. Never mind the quality, look how many channels there are.
In a typical living room for average vision with decent content, 1920 x 1080 is OK up to about 50" to 60"
UHD needs minimum 120" to be worth while for average viewer. Pointless on 42"
Increasing frame rate to 96 fps (x4 cinema) would give more significant improvement at 60". Perhaps native at 90, 96 and 100 progressive would allow good quality interpolation from existing sources.
Interlace was a 1930s analog trick to half the transmission bandwidth by temporal compression (still at 25i looks similar resolution to 50p (50fps) still, the eye can't see detail as well on movement, which is drops to 1/2 vertical resolution.