What you just described isn't a problem that the vast majority of people are having.
Apple isn't going to cannibalize their own product lines by combining two items into one to satisfy a small subset of people who absolutely need to have an iPad mini on them at all times.
Foldable phones are a solution in search of a problem, and IF Apple ever does come out with one, and I don't believe they will, it's not going to be anything like the ones on the market now, because they're unimaginative garbage.