Yeah it’s the plastic around the hinge that breaks on the DS, it was much worse on the originals than on the DS lites.
The biggest issue with the folding phones though is that folding the screen in half is going to cause wear and tear on the display and it makes it easier for it to break along the seam. Only way I could see a folding phone working is if it had two separate screens like a DS/3DS.
I don’t have an issue with phones being too big for pockets since I have plenty of room in my pockets, even for relatively large phones.