So, TL/DR
the iPad market is saturated with devices still working just fine! Including the earliest models, not to mention continued support by Apple for the older devices has made them much more like a computer investment than the personal investment we think of our smartphones
I don't believe they're going anywhere and I feel like the opposite is true. As more folks use hand me down units, used or pre owned tablets like iPad - once they have a chance to play with a current iPad, they'll be hooked -- as the time comes to start replacing essentially the 'first run' in human history of usable computers in tablet form and we're seeing it's infancy
Desktops are nearly out, other than AIOs
Laptops are the focus of the OEMs but 'convertible', hybrid and '2 in 1' are the 'ultrabook' buzzwords - all trying to achieve parity with perfection that is iPad and iPhone married with the App Store and an OS X/macOS counterpart at main base - no problems