For me it's a better product than carrying two different items. If I have to take a tablet I have to bring a bag/case to carry it, and then unzip all that to pull out the tablet every time I want to use it (e.g. video, comics, larger Google Maps). Chances are I just won't bother. It would be similar to taking a laptop with me.
With a foldable phone, I just unfold it. The convenience leads to using it more and that's a big advantage.
Right now the crease/reliability is a worry, but assuming that is resolved I'd pay $1800 for it.