I do believe Apple will release a folding iPhone in due time. I do believe there is a market for such a device.Not really.
Folding phones sound and look cool for demo, but in reality, it’s cumbersome and cannot be a great phone nor tablet. It would be too thick and less durable for a phone, and it would be too cumbersome for a tablet.
The foldable phones are simply demos for the technology, which will be used more subtly in existing products. Eg. Apple already use foldable OLED on the X/XS as a way to minimize the bezel and prevent the phone from having a chin.
The biggest question I have is the thickness. Keep in mind however, the Mate X when folded is thinner than the original iPhone. It does however have a much larger footprint, so I really can't say whether it would be comfortable to carry at these current dimensional specs in a front pocket. Perhaps Apple can offer something thinner when folded if/when they release their own version.
If Apple can offer something that is manageable in a front pocket, then I would purchase on day one. I would love to be able to fold out my iPhone as an 8" screen.