Apple has never said that iPad will be the primary device of the future. That is a conclusion some iPad users have made on their own. Apple has, however, stated the opposite—that Macs and iPads will both continue to have their place, only that which device one chooses depends on the user.
The reason both will continue side by side is that iOS, with its sandboxed app-centric os, touch ui, and corresponding applications, was designed and is perfect for consumption and shorter input interactions. MacOS, with its file-centric os, pointer ui, and it’s correspond applications, was designed and is perfect for more demanding workflows and longer input interactions.
One is not better or more advanced than the other. They’re simply made for different purposes (insert car+truck analogy). Apple’s push for iPads to replace more laptops simply reflects the fact that for some who are currently on MacOS (or preferably Windows), iOS is better suited for their needs (especially with the help of the keyboard and pencil). It was a marketing push in a time of slowing sales.
It’s possible iOS could pivot to become file-centric and have mouse support and thereby push out MacOS, but at that point, what’s the major difference between the OSes? One could accomplish the same thing the other way, by adapting MacOS for touch. But both are extremely doubtful (outside of the rumor about mouse accessibility support in iOS). Apple has been very clear about maintaining both platforms.