>match the fidelity of the sub $100 4K monitor

This can

>8k per eye

Not required to get 4K fidelity… you only need somewhat higher than 4K resolution.

I'm not talking about "4K fidelity", I'm talking about "the fidelity of viewing this 4K monitor", which is different. When I'm looking at my monitor in front of me, it fills a small fraction of my visual field (around 15–25%?). So for a headset that covers nearly the entire visual field, the resolution would need to be much higher in order to reproduce the pixel density of the monitor.

This is, for most people, pointless, as the rendering costs will be much, much higher (>8x for a quadrupling in resolution per eye), they probably won't care if text is not quite as sharp as a real 4K display, and they might not even be able to tell if their eyes haven't been trained on them.

Unfortunately though, I require a 4K display for sharp text (I get really weird visual effects from low-resolution displays), and my current HMD (HP Reverb G2 at 2160x2160 per eye) is uncomfortably blurry, so I don't imagine the Vision Pro at an estimated 3400 pixels per eye according to their marketing figure (`sqrt(23000000 / 2)`) can do much better.

I know it's revolutionary and this is about at the current limits of HMD technology, I'm not at all claiming that it's not impressive for the industry. It's just part of the reason why I don't use HMDs often is because the tech hasn't yet advanced enough to be comfortable for me.