Is there any reason they'll remain exotic, expensive and not that small? (I don't know what interface the typical high-dpi oled touch screens use, like the ones you find on a Galaxy Note 4 etc) -- but it seems that HDMI might make perfect sense for a tiny 4k screen (and it seems reasonable those will become more and more common)?
Devices don't use HDMI to talk to their built-in screens due to licensing fees, complexity, etc. Usually it's either LVDS or embedded DisplayPort these days, with newer/4K screens being more likely to use DisplayPort and older ones generally using LVDS.
