Now that we are seeing the first folding smartphones coming out, I feel fundamentally pessimistic about them. Unfolding will always take an extra second or two, and that is problematic for quick checks.
As long as the phone remains our primary communication device, we will value quick access a lot. So either another device must replace the phone. The watch and AR are the best candidates for now, but both are still far away from being capable devices for responding. Or the folding phone must work very well both folded and unfolded. This is a challenge, with the extra weight and space required, and with the screen the largest battery drain. I am afraid that folding phones will need to add too much bulk to stay competitive with normal phones, especially now that the standard size has become large enough to comfortably read even longish texts. The declining sales of the iPad mini give a strong indication that the expanded state would not be valued enough to justify costs.
I am much more optimistic about folding tablets. These are already devices that are used for longer sessions so the folding will be much more tolerable, and it provides a real benefit in making tablets more portable. We will have to wait a bit though until the technology has caught up to allow large, durable, folding screens with good enough power consumption.