If I can hear quantization or distortion in my speaker system, it's unacceptable. It should, in the least, be able to beat the somewhat small bandwidth of human hearing, which can be accomplished with pretty cheap speaker systems.

Being able to see pixel-wide features isn't the point above witch nothing is gained, it's the bare minimum below which quality drops precipitously. In the least, you'd want to double that resolution, on each axis, to guarantee that patterns don't create quantization errors, similar to the Nyquist cutoff in audio. It's also worth aiming for at least the 90th percentile in visual acuity, if not the 99th, to ensure everyone gets the best experience.

Historically, good video quality has been expensive, but as the price comes down, there's no reason to not take advantage of it, just because some won't benefit from it.

Some people listen to music on their phone's loudspeaker, but that doesn't mean concert halls shouldn't aim for anything better.

This is true in theory but in practice, you can see the difference. My mom just came to visit from out of state yesterday. I recently added an Nvidia Shield to my Roku 4K TV that upscales the content to 4K. The first thing Mom said when she turned the TV on was, "Did you get a new television?"