A foldable iPhone isn't the future, but a folding iPad or MacBook might be

~3" wide is the practical maximum for a phone. It's been about 7 years since the 6 Plus, and longer for Android phones, and phones that are wider than about 3" have not been successful. Or have not been successful enough to drive change in the market, other than being just a niche market entry. Android phones have definitely tried 7", 8" class phones. Just ended up being too unwieldy for hands and pockets, or the increase in screen size was not enough gain to overcome the inconvenience of a bigger and heavier device.

This all screams "small niche" as the potential mass market user gains in folding handhelds aren't worth the losses.

An all-screen laptop, 14" to 16" half-display size, has a benefit of giving a user an 18" to 22" display when at a desk. You have to use a software keyboard and trackpad while on the go or in clamshell mode, but I'm absolutely fine with that as that's how I've been using my iPP10.5 for over 4 years nows. My primary use case for my MBP15: open with 1 cable to dock plus external display at work, open with 1 cable to external display at home, and with external keyboard and mouse in both places. It's in a backpack to and fro. I use it in a clamshell mode with its keyboard maybe 0.1% of the time.

So, all-screen laptop gives me more screen space for my work and that's a big plus. So, the all-screen laptop or folding 20" iPad rumor is much more interesting to me. For mobile, handheld devices, seems like a niche for now.