"Screens" (in general, leaving the Mac out of it for the moment) are not limited to the headset's resolution (which is, in any case, ~4.5k per eye, so somewhat more than that for both eyes together). They're virtual objects in space, and being virtual you can look at them close or from far away. And you can in principle zoom in or out as well.
The actual limitation on Mac screens will likely come from bandwidth between the AVP and Mac, and whatever protocol they're using to transport frames. So in practice, especially for video and games, 4k may be too big. Or maybe not, wifi 6e can move a lot of bits over 6ghz.
As it happens, for the moment it seems like you're limited to a single Mac screen, but there's nothing about the Mac or AVP that creates that limit. Apple could expand that number tomorrow if they felt like it. I suspect that over time they will. At that time they could easily support multiple 4k screens- since they know where your attention is, they can focus their rendering and bandwidth resources where they matter, doing low-res versions on the screens your eyes aren't focussed on. That is after all what they're doing in general with the AVP (look up "foveated rendering" for more info).
Your certainty about this doesn't make you right. And your lack of historical perspective is overwhelming. Dedicated hardware of all sorts has been overtaken by general hardware with more favorable economies of scale for decades.
Think the iPad was considered an "enterprise product" when it was released? It's definitely a consumer product. It also sells a ton to businesses.