The issue with repairability is always the conflict between water resistance, thickness and feel vs the compromises to that needed to make removable batteries or back cases work. There are ways around it but the vast majority of the market prefers solid near glued together phones so that's what companies make.

> I get the impression they're running out of ideas with the "slate" form factor and are trying to stimulate sales.

I think we've just reached the local maximum of the phone design and adding folding gives two different branches to go down: 1) same size screen unfolded but smaller folded size or 2) folded with an external screen roughly the same size as a normal phone today but it expands to a much larger one unfolded. We'd kind of reached the peak size that people can reasonably pocket so option 2 allows for even bigger screens for people willing to pay without having to have a second device (something like a tablet).

The folds do add functionality and I think there's an impulse that leads people to say they don't see the point of something just because they're not interested in it personally.