The thing I like about the iPad Mini is that it's an optional carry. I don't need an 8" device every time I leave the house. And maybe that's going to be the appeal of the foldable.
I also don't foresee ipad minis going down that much in size, whereas foldables are constantly being made in smaller sizes (in height, thickness, weight, and even a variety of aspect ratios).
Price is the final real hurdle for most of these things IMO.