For those of us who lived through the age of cell phones that had either a folding case design or a plastic covered screen, these are the two reasons why you would doubt the future being folding devices. A mechanical hinge is a failure point, not an if it fails issue, but when it fails, because it will.
And even with advancements in foldable glass, which really aren’t advancements, but just using ultra-thin glass, the longevity of it is another failure point. Add in the fact that in both cases, smart phones / devices today get used far more every day than cell phones did back in the ’90’s, means the potential for dust, sand, scratches, drops, etc. are so great that Apple would be risking brand value to release a device that can’t survive long-term.