Another benefit of the foldable over the ipad mini + regular phone is managing internet connectivity is a lot easier. Also no need for 2 separate sessions on whatever apps/websites you're currently using. As much as syncing states works fine nowadays, it'll never match just sticking to one device and not having separate browsers / note taking apps for your phone + the ipad.
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.
I thought part of the point was that the iPad would _not_ have internet connectivity. Nothing to distract you. No notifications to check.
In general, managing a single device is always going to be easier than managing multiple -- fewer things to keep charged, fewer updates to run, less stuff to carry, etc.