I get what you're saying. I just think if we ever end up with a wrist-worn device that needs a display bigger than itself, we're going to have some sort of projection trick going on along with gesture recognition rather than a physical expandable screen.
You know, tilt your watch toward you and "see" a larger "screen" hovering over it. Interact with controls by "tapping" them, tracked by depth cameras in the watch. focused puffs of air for tactile feedback maybe.
Seems out there, but given the hurdles materials science needs to get over to give us durable foldable flexible displays that will survive the beating they will take on our wrists, I bet we're looking at about the same timeline. Say, 30 years or so, assuming no nuclear war or worldwide economic collapse.