I say the same thing about app developers when I use my iPad.You bring up many valid points. That said, there's no reason that apps can't be adjusted to take advantage of the extra screen space (wouldn't be the first time, and certainly wouldn't be the last either).
The way I look at it - a folding phone sells the potential of a convergence computing device. I might be able to have a single portable device to replace a phone, a tablet, heck maybe a laptop.There are many use cases where extra screen space would be very helpful. When I think about the apps I use most - Maps, Mail, Messages, GoodNotes, Banktivity, Gaia GPS, Photos, Anybox, Calendar - all of them would benefit from the extra screen real estate. Multi-tasking would also be a lot nicer on a bigger/wider screen. You could have two full-size apps side by side.
But there are a _ton_ of trade-offs there. It's not clear how you have either 'state' be as good as say a dedicated phone or a dedicated tablet. Usually both forms are significantly worse. And there's not really been any clear compelling cases for it outside of comparisons to those form factors.
So once flip phones no longer have a cool factor, I'm really curious whether convergence wins out over the experiences two dedicated devices provide, or if people lose interest because if they want a larger screen they always have their tablet/chromebook/laptop/desktop/whatever. Maybe it does win out simply because it is a way to fit more screen real estate into a pocket - although that means we'll might start seeing tri-folders and quad-folders ;-)