But, they aren't doing fovea tracking and shifting the display around in response to eye movements, are they? If not, you would only get more screen space if you articulate your neck and cause the head-tracking to update your viewing perspective.

I've monitored how I use 4K screens in the 28" size range. I would not normally move my head at all to look at different parts of the screen. I would only move my eyes. With two such screens side-by-side, I would turn my head just a little bit to focus on one screen or the other. Or, I found I would often hold one head position that is neutral between the two, and then use approximately 2/3 of each screen, with the far outer edges neglected or used to banish less relevant communications and status apps that I only check once in a while.

And, these screens are not filling my field of view by any means. So, I'd really need a far higher resolution headset if it is going to give me a good immersive field of view and be a reasonable monitor-replacement. I fear that fovea-tracking will remain scifi dreams in my lifetime, so the reality is we need to render full resolution throughout the field of view to be prepared for where the eye gaze might go in the next frame.