I'd even go so far as to say the idea is the easy part. Tablet computers are an old idea, they had them on Star Trek TNG. Engineers have been trying to make the PADD a reality pretty much since the first episode. iPad wasn't the first attempt, it was the last.
It wasn't even Apple's first. And unlike the ones that came before it actually did reasonably well at matching the depiction. It got the relative size, weight and pick up and just use experience.
But nobody at Apple would have known the tech was ready if people hadn't been trying over and over to do it for the previous 20 years.
New tech is almost never an original invention that came from nowhere. The internet was built on decades of research specifically into networking computers together but also built on almost a century's worth of development into the phone network. The experiments that lead to radar date to not long after radio became practical. The Wright bro's weren't the first to try an airplane, they were the first to make it work. and so on.
Its the implementation that matters.