Personally I find several times a month that I would prefer if my phone could unfold into a larger view vs carrying an iPad, and I really like my iPad. I think there is some gas in the tank for this idea, but you are likely correct about it being niche. I always want more space in Maps when I'm out exploring new places, for example... but don't always want to lug the iPad.
The Z Flip is form over function though, I've seen one of them in person and they're far too tall and narrow when unfolded, and don't really make up for much by being able to become shorter and thicker in the pocket.