A screen that folds quickly without breaking is still a huge open R&D question.
That investment is not cheap.
I think we have avoided the step where the military or the industry wants it first and is willing to give way to them, and it “goes” to consumer technology. Somehow I’m personally financing their R&D department for a dang call and text machine.
(Sidebar / Rent for the rest of the post …: GPS navigation is the only argument for owning a smartphone that I would personally support. There is no need to rediscover the black rectangle for GPS … Folding doesn’t help …)
Oh and smartphones, especially Android, require huge batteries to stay awake. Flip phones require a segmented battery (or a half-sized one) and wires spread across the hinges. High cost, probably low overall battery. In any case it is difficult to design in a work fashion.
All compromises on a flip smartphone, virtually no gain. Must be seen when not being used without being different and neat.
If you want a real flip phone, get one without the modern app. Let the smartphone be brick. These are already small surprise-devices, if you don’t mind how overloaded IMO they are. I say use a real computer when you need a computer. Get a dumbfound for calls and text unless you specifically need GPS navigation. But if you want to go smartphone, brick layout is tried and true, more durable, simpler design. Better …)