There are also those that say that unless you're watching a really large screen you won't be able to see the difference. I'll simply reply to that with the observation that I've seen enough 4K demos at trade shows and conferences that this is nonsense.
I can't help but think that some factor of this might be non-ideal equipment/content. This does not make your observation any less (or more) accurate, but it might explain the apparent descrepancy between theory and practice: while 720p might be theoretically "sufficient" for a given display size/viewer distance, once you factor in Bayer camera sensor, multiple passes of scaling/sharpening, suboptimal lossy coding, display pixel shape, etc, simply moving to higher resolution for every component might "shift" those suboptimal characteristics into higher frequencies (that are less visible).
Perhaps (many of) the same quality gains could be had at 720p if every manufacturer followed "best practice", but perhaps those same manufacturers have more to gain by branding improved equipment/content as "UltraHD"?
Delivery of 4K content is also an issue. There simply isn't the bandwidth available for 4K on current cable or satellite systems. Not unless you want to cut the number of channels available by 75%. Broadcast bandwidth is an expensive commodity.
One might expect/hope progress in video codecs to continue for some time (improved per-pixel quality per unit of bandwidth). This means that one can either increase the number of channels, or the quality (e.g. resolution) of each channel, for a fixed total bandwidth.
It seems that market mechanisms favours increasing the number of (silly) channels, rather than increasing the quality over some minimum quality. They will happily sell "HD" channels, but at the same time constrain bandwidth so that the effective quality is comparable to SD.
I have been subscribing to a realtime webbased streaming solution (from HBO), but the quality and robustness is far from my traditional fibre-based cable box. It does give me "Game of Thrones" episodes within 24h instead of 1 year, which just shows that content is king, I guess.
It seems to me that the pirates have found the solution: P2P. If Sony/Apple & friends did P2P (using each customers PC/PVR as a proxy), then distribution of very large files seems to go effortless and inexpensive, exploiting unused bandwidth/storage for each user. It might even work for "semi-realtime" playback of sports and news. The actual payload could be encrypted or whatever is needed in order for the content providers to demand my money.
I think that other aspects of the "ultra-HD" spec are just as interesting as spatial resolution: 120fps, ITU BT 2020 color space, 12 bits per primary. It remains to be seens how much of those specs will be actually utilized in shipping equipment and media. With todays tvs offering very large DR (bragging about even more), one might hope for a system that allows capturing and delivering content to suit.