This is kind of where I was. iPhone (mini was my fav) + iPad mini. I would use the iPad mini for when I need to read work documents, those times I needed a slightly bigger screen. When I got my 17 Pro I just stopped using my iPad mini as much, not because I felt the 17 Pro screen was big enough for my needs but just because I didn't want to carry both while on the go. For me I think iPhone Duo could solve this... maybeI guess the wager is all these people who wanted an iPad Mini but didn't buy one, as they only want it if it can unfold into one on the go?
I'm sure there are many who may want that ...
But it makes for a very heavy brick in your pocket, for the price of a full blown really nice MacBook Air 15".
Nothing is really for "free" here, even on foldables.
You get the smallest iPad when you unfold, and a heavy brick of an iPhone, missing some key things, the rest of the time.