What you state is 100% valid by now, and for sure that at this time the market is small mainly because Apple hasn’t entered to the game yet.
I can predict that the changing trend that is coming is just comparable to when we abandoned physical keyboard devices in favor of all-screen smartphones, even although a lot of MR posters don’t like the foldable device idea by now.
Meh, it would need to price-comparable to a standard phone for it to be that mainstream, even if people did want them, which I'm not sure the majority does, from any manufacturer. The people I've asked about it are Android users..they are ones that don't want one, even after trying them out.
My maximum price for a phone is about $1200. I can easily afford more, but smartphones just aren't worth more than that for me. I can't see Apple releasing it in that price range.
It's fine for flip-phones to stick to being a high-end enthusiasts device. There is nothing wrong with that.