I disagree. It's not that Apple is not able to make a seamless iOS/macOS hybrid, it is that they have no desire to. Currently, they're positioned very well to selling more than 1 computing device to customers. I'm pretty sure that many who own an Apple notebook also own an Apple tablet. These customers represent a dual stream of income for them. What incentive do they have to combine those two?
The "seamlessness" that you refer to is BETWEEN Apple products, not merging them all.
I'm calling it here and now, I believe that Apple will ONLY produce such a hybrid device IF their competition is able to produce one that poses a viable challenger to their existing product line. Apple held off on the iPad Mini until Android tablet makers started selling 7-8" tablets in significant numbers. Apple held off producing a phablet until Android smartphone makers started selling phablets in significant numbers. The most effective way to get Apple to produce such an iOS/macOS hybrid, is to buy a competing product like a convertible chromebook or 2-in-1 Windows device. (except for the Surface... THAT would be shooting yourself in the foot)
In the meantime, Apple will do what Apple does, disparage the competition that is offering something that they don't. They disparaged phablets and smaller tablets until they were forced to deal with the reality of their popularity.
I agree. See above as for WHEN they would produce such a device.