Same ******** based on advertisement and emptiness: 99% of my friends, colleagues, or other workers still use PC aka "Personal Computer" no matter it's shape or form, you're pulling arguments out of your bottom, Steve Jobs never mentioned desktop you're just inventing stuff to stick to your "post-pc" narrative.
Desktop will simply change in smaller form-factors (like the Mac Pro) because professional in any science, entertainment, development, finance, game etc...domains need a powerful graphic-cars which laptops will never have, while others will stick to the laptop as long as it has a physical keyboard, trackpad and most importantly a serious OS.
It's not just iPad sales that are declining, it's tablet overall, and nobody uses smartphone for other than communication, quick informations and gadgets. We are still in the PC-era, and when screens have become so thin that they're transportable or foldable, maybe Tablet will become a "controller" (haptic keyboard and trackpad) and secondary screen of other devices, but PC not matter it's size will still be the "control center" of experience and work.
The tablet in it's current form as absolutely no use between the PC and smartphone, in fact even professionals who've used tablets for some times, are not ordering them anymore because it's useless, the same goes for universities when students will start having to learn coding, science or do serious work, the best a tablet might become is what it is now: a secondary touchable interface.