You are missing the step where compact laptops became cost competitive and powerful enough that they weren't a burden to travel with. This made them an extra device for many people.
There was a time when everyone got these unwieldy and clunky laptops, that turned out to be as portable as a desktop under everyday circumstances. So when they needed something actually portable, they went out and got netbooks. Then later, as netbooks went out of style and the iPad was all the rage, they went out and got tablets.
But today, if you're lucky enough to have a laptop that's actually conveniently portable, a tablet doesn't seem to have much use anymore. A good smartphone has the advantage of being on your person all the time. And a good laptop can basically do anything. So what exactly do I need a tablet for? Its not going to be on my person as regularly as a smartphone, and its not going to be as capable as a laptop (that's now similar enough in size/weight).
That being said, there do seem to be a lot of older (non-computer) people who have somehow replaced nearly all of their "outside the office" computer use with iPads.