I think many people would happily go for a flip phone when the tech is more refined and advanced, and cheaper - most hatred seems to be focused on the crease, or the lower specs, or vulnerability of the hinge, and on the price. These are issues that will all be solved as the market matures.
Book style foldables will likely have a smaller market as they are solving a problem only a limited number of people have. I'm probably in this bracket, but I'm not willing to pay the prices being charged at the moment.
Foldable tablets only address a niche demand, and it's likely to stay that way for quite some time.
That said, foldable tablets, where the financial risk is likely to be small and the engineering is likely to be somewhat less complicated, would be a very good place to prove the concepts and develop the skills prior to moving into phones. I'm unlikely to buy a foldable iPad as the price is probably going to be horrendously expensive, but I welcome it from the development point of view.