This. The tablet was held back for nearly a decade by Intel and Microsoft insisting that it had to be a convertible laptop. Microsoft wanted to make sure each tablet sales was a Windows license sale, and Office too if they could. Intel wanted to make sure each tablet sale was was an x86 CPU sale, and a high-end CPU too if they could. Consequently, the tablet PC market stagnated at fewer than 100,000 sales per year for close to a decade.
The real technology that led up to tablet market space wasn't the smartphone; it was the netbook. Suddenly people realized that most of the stuff they did on laptops (email, web browsing, myspace/facebook, listening to music, watching movies), they could do just fine on devices which didn't run Windows and didn't have a PC-like CPU, and consequently could be cheaper than a laptop, not more expensive like tablet PCs were.