As much as I want to agree with you, those old phones had low resolution, low refresh rate screens. You need a much higher quality connection to push UHD video

As much as I want to agree with you, those old phones had low resolution, low refresh rate screens. You need a much higher quality connection to push UHD video down, and I doubt you can get a slip-ring to reliably carry signals of that high a frequency. The capacitance and signal reflection issues would be tricky to work around.