I laugh at people buying 4K TVs… your eyes are almost certainly not good enough to tell the difference in the vast majority of viewing scenarios. Total waste of money
If we were just talking about the absolute resolution, you are right. At my normal viewing distance I can’t tell the difference between 720 and 1080. I’m certainly not going to see much improvement from a 4k screen. But it isn’t just the resolution. The more pixels you have to display the image data, the better the image looks. The pixels start to disappear and be truly invisible. I’ve known this about printing for years, but when you apply it to video, it really helps the realism of the image, even if you are watching a 480 DVD. With the proper hardware, everything will look better on a 4k screen, even if you never get a UHD player or media.
My dad has a 43 inch 4K TV and DVDs look horrendous.