That's just the point. The need for desktops/laptops has been going down.
The growth of PCs is way off. There isn't much evidence that the need is way down. The replacement cycles are also getting longer. Again that isn't a measure of "need" but rather of "buying frequency".
What folks don't "need' to do is buying a legacy PC form factor when didn't really need one in the first place. Where have a limited set of tasks to do a more limited device would suffice a more cost effective device will do. Actually the real need was not matched to the legacy PC.... it was really just the closed kludge to getting the issue solved.
Why not migrate the hardware of some of these form factors closer to the market success, where's there's tons of current software and system investments, mobile devices?
A growing faction of mobile devices are not particularly mobile. A household that needs a 2nd "Person computer" so two people can surf the web at the same time may buy an iPad (or lower cost tablet). It is pretty cost effective having to buy another PC+Screen+windows. It can "float" around the house but doesn't necessarily have to leave. (very similar trend happened with cheaper laptops. )
If the usages are limited relative to the hardware's capabilities there isn't alot of upside to computer being big. ( impart that's how transitions from Mainframes to "Mini computers" (which were still usually refrigerator or bigger sized) to Personal computers ).
iPads are used in some classroom now because they are "good enough" for the software that needs to be run and can be put away when done and have other things to do with the desktop space. Traveling large distances or even connecting to different networks isn't really a big issue.
The CPU architecture should no longer be the differentiator. Just UI/UX.
CPU architecture is huge differentiator in making existing software work well. Inertia is pragmatically a factor. System vendors like Apple buy architecture implementations; not architectures in general. The x86 architecture isn't necessarily better but the current and immediate future implementations are certainly far more suited for the more than just occasionally plugged in computer systems running broad spectrum personal computer workloads on large, bulk local data collections.
Significantly chop down the I/O demands and pragmatically solely run on limited battery and ARM's implementations/instantiations have some more competitive upsides.
One for a tiny mouse pointer and huge (dual) monitors while sitting in a comfy desk chair. Another for a tiny pocket-sized display and fat fingers on a bumpy train ride.
iOS transitioned past the "tiny pocket sized display" stage years ago. In fact, the general notion of iPad started before iPhone. That "tiny pocket sized display" was only an initial jump starter mechanism.
The big dual monitor set up is only a smaller subset of the OS X space. There is a substantive fraction on the "lower end" that can overlap iOS. Much of the lower end cannibalization is already there without the form factor overlap.
iOS can pick up additional form factors far more so because the market is large enough to support different subgroups with different products targeted to their needs. If there is a significant fraction of users buying iPad+keyboard then it can very well make sense for Apple to sell the combo as a completed system.
That's far different from the trend line that classic Mac OS form factors are on. As pointed above the growth is stalling so there isn't a new larger group. OS X isn't chasing the iPad slate form factor just because "it's hot".