I presume you're already aware, but for those who aren't: MacOS already has Sidecar, which lets you use an iPad as an additional screen.

If I understand you correctly, you're talking about a really big iPad that could be used as a normal-sized monitor, but folds up to the size of a regular iPad. I too think that would be amazing — but more so because it would be a step in the direction of a tabletop- or whiteboard-sized iPad. I would love a device that large for manipulating mindmaps, outlines, etc. I suppose, though, that Apple Vision Pro might obviate such a thing by projecting a huge virtual monitor on the wall, say. Still, I like the physicality of manipulating objects on the iPad (like nodes on mindmaps and outlines).