It depends on the size of the screen. I have a 32” screen and 1080p looks a little pixelated at arms length; about my normal distance when I am using it as a computer monitor. As many home theater systems have screens considerably larger than me (about 69 pixels per inch), a 1080p image will look “jaggy” at longer distances as each pixel gets larger.
When it comes to computing, I am relatively casual compared to those who do things like hardcore CAD/CAM or professional image editing, and while I am a gamer, I’m not the type to spend a couple of grand to have a pair of top-tier video cards in SLI just so I can run around on Maximum at more frames per second than the screen can draw. In other words, even though my demands are a little higher than most people, even I’m not geeky enough to be part of the target audience for this.
But a lot of people are using a PC as a home theater system, and like I said before, many of those have huge screens; 50 inches or more. If you take the same number of pixels I have and try to cover three times the area, those pixels will grow quite a bit, so it makes sense to have higher resolutions… but only on large screens. My 69ppi is alright for me, but I really wouldn’t want a monitor that had less than 60ppi and those fussier than me would want something with the pixel density of the average smartphone (over 300ppi) to look more like analog film with effectively infinite pixels per inch.
So it’s not just the photographers, movie editors, and hardcore gamers that would have a use for 5k resolution. Anyone who uses their computer to watch videos on a flatscreen that’s 40 inches or larger has some skin in the game as well. Now, how many people have both large TVs and either Netflix or cable? Enough to make 5k financially feasible to produce as there will be enough demand to turn a profit.