The point of a foldable is that you have a device that still fits in your pocket, but then the screen is twice as large. But I only want a larger screen for video, or a game, for instance, which for most people is a minority of the time.
Meanwhile, using it the way we normally use a phone most of the time would be worse because I'd take it out of my pocket and the screen is not visible until I open it. When I do open it, I can't use it physically the way I always used a phone - mostly one handed. Think about all the times you look at your phone for just a second or a few. Do you want to unfold it each time?
So with a foldable, it's one step forward for large screen content, and two steps back for most content and functions, most of the time.
If the phone folds such that the screen is on the outside, I won't have to open it, but the phone will be twice as thick while using it most of the time with one hand, for regular stuff. The aspect ratio is dumb, and the aspect ratio sucks. And you'd have to raise it up with the correct side facing you or FaceID won't work. Another design it a screen on the outside and two screens on the inside. This is what the Oppo is. Thick as a brick... for most of the time you are using it, folded.
Ok, forget a foldable iPhone, what about a foldable iPad? I only ignore my iPhone and reach for my iPad for the things I want a big screen for. It is sitting on my table. Why do I need it to be folded, so it can fit in my pocket? No. So it can fit in my backpack? No. Is it so it can have a screen on half of it and a keyboard on the other half? That's a laptop.
But what about an iPad that has a 16 inch display? It would only be half as large when folded (and not being used sitting on my table). Who cares? Do you want to hold a 16 inch iPad in your hands? It's not going to happen. What about folding a 13 inch iPad Pro? Why would you fold it? So it can fit in your pocket? It still wouldn't. Is it so it would fit in your backpack easier? It would be a smaller rectangle but twice as thick, so the bulk would be the same. The only bag that is that small is a purse. Does a woman want to carry around a foldable iPad in her purse AND her phone?
Do you want your display surface a flexible plastic? I want glass.
A foldable is only interesting because it's amazing that a screen (a plastic one) can fold, but I don't actually want it. It's like those tablets in the movies that are a clear sheet of glass you can see through. Do you want to look at anything on the screen, and be able to see everything behind it distracting you? No one wants it.
People buy foldables so they can show them off, but they suck to use.