Many commenters are pointing out that your post is opinion. It is, although you deny it. You never defined "flop" in a way that can be tested (X% of iPhone shipments, fiscal loss for Apple, market share retention, etc.), you made a strawman argument ("there will be a small brigade"), you make absolute claims ("no productivity gain", "solves nothing", "nobody is asking for this") without any clear definitions or evidence, you assume the "slab phone" is perfected (if so, why does Apple or Samsung or any company make any changes to them year to year -- only to sell new ones or because they figure out ways to improve on what's already there?), you assume that because iPhones and iPads already exist there's no benefit from a single device, etc.
It's fine if you don't like the idea of a foldable iPhone. It's fine to think it will be a niche product. However, what you wrote is more akin to an op-ed than something objective.