I think that this is two different issues.

Foldable — I expect that the attractions of a foldable phone are larger screen that fits into a pocket; better protection for the screen; and the satisfying action of closing to end a call.

Cast to a nearby screen — this is a good idea for any type of phone (or device) but would be limited to the walled garden of your phone manufacturer/software provider.

Another Alternative, Currently, foldble phones gaenerally fold in half, like opening a book halfway. Alternatively, you can choose to fold just the screen. Similar to a laptop, where only the thin screen is opened, it’s like a book where only the front page is opened and then closed again.