This still feels like a solution in search of a problem.
A foldable MacBook? All MacBooks are foldable. If you mean a MacBook with touchscreen instead of a keyboard then that's just a worse MacBook. Remember how popular the Touch Bar was? Now imagine that for the whole keyboard.
An iPad? Sorta seems like it might be useful at first glance, but if you think about it, not really. So I can use it folded up, using the outside screen (which is usually compromised in some way on competitors devices), but then I'm holding a device that's twice as thick, bulky and heavy as a normal device that size. Or I can open it up, and have a device that's mostly the same as a non-folding iPad that size, except it has a crease, decreased longevity and resilience, and has whatever other compromises were necessitated by the hinge and second screen (presumably less battery life, or it has to be thicker and heavier to fit the same amount of battery in). And what have I gained? It's (arguably) easier to store? No thanks.
An iPhone? Pocketability might be improved I suppose, if the device gets half as long but twice as thick? I dunno, I use a Mini so it fits my pocket fine anyway. I'd have thought that added thickness might be more of a hindrance than a boon when it comes to pocketability but I don't really know.
The only real value-add I can see here is the kind that are normal phone size when folded (so remain pocketable) but fold out to small iPad size. That could be useful. Not sure it's worth the compromises, but maybe.