why would apple release one? demand seems nonexistent.

Samsung has released their 5th generation folding phones and they’re still demonstrably unpopular. There is no evidence of any broad demand for a folding phone. Apple doesn’t make niche iPhones.

My theory is Apple won’t release a folding phone until at the very least, the durability and feel of the screen material is addressed. In addition, I personally don’t think Apple will do a folding phone design until they find an implementation that does more than replicate the standard flip phone or just expand the screen real estate.