It is true that few viewers will see the increased spatial resolution of 4K without expensive large screens, and even less so for 8K.
But increased spatial resolution is far from the only change in the the new UHD Premium broadcast standards ("4K") or the Super Hi-Vision ("8K") standard mentioned in this story, and all of these other changes have a huge impact that the normal viewer can easily see, no matter what size the TV.
Firstly there is increased colour gamut - before UHD Premium the standard was the BT. 709 colour space, which could only mathematically represent a small subset of the colours that human vision can see. (For example, Pantone's colour of the year 2013, Pantone Emerald 17-5641, can not be represented in BT. 709). The new broadcast standard enlarge the colour space to BT. 2020, which is much larger. Current devices can display roughly the DCI-P3 (digital cinema) colour space, which is already larger than BT. 709. BT. 2020 provides future room for growth beyond even that.
Secondly, there is increased bit depth. Instead of using 8 bits per channel, UHD Premium provides for 10 or 12 bits, and Super Hi-Vision requires 12. 8 bits per channel is just not enough for shadow detail (have you noticed how many videos just have a splotchery of large dark gray blocks in the shadow regions), and causes banding and posterisation in areas of smooth colour gradation like skies and sunsets. (Blu-rays mitigate this by having human compressionists continuously adapt the compression algorithm's parameters on a per-scene basis to work around this).
Then there is increased dynamic range. The current range of brightnesses BT. 709 supports can roughly be described as "everything from black to a glossy white paper under sunlight". Real-world scenes go beyond that, i.e. specular highlights on metal, or the heart of flames, and UHD Premium and Super Hi-Vision provides for that. The idea is not that the entire scene should be brightened to the point that you suntan in front of your TV, but that detail should not be lost by clipping specular highlights, flames, sunsets, full moons and the like.
Then there is increased temporal resolution: UHD Premium and Super Hi-Vision allow frame rates up to 120fps. This will make a large difference in sports with fast-moving objects, it gives additional creative freedom to directors of photography should they decide their stories are better told with higher frame rates.