It seems theoretically possible, does anyone know if it can happen?
Also take in to account you have weight on your head. In games, players often get a sort of tunnel vision and ignore whats going on around them, making it easy to ignore the lack of comfort.
For an HMD you only need "retina" resolution straight ahead. Peripheral vision can make do with a much lower resolution. I have no idea what the magic number is for retina HMD, but it'll probably be smaller than 12 million pixels. It's certainly possible that the 5" 1080p display used in the latest HTC could have a higher apparent resolution than putting 3 WQHD monitors on your desk.
And also keep in mind that higher densities are much easier to manufacture affordably in smaller screen sizes. There's a reason why you can buy a phone with a 440PPI, laptop displays with 220PPI but it's hard to buy a desktop monitor with more than 110PPI.
"Straight ahead" depends on where your eyes choose to focus at any one moment.
He says that for full immersion you need an 120 degrees horizontal and 120 degress vertical 8k by 8k screen