If you don't recall PalmOS, the major user interface was a resistive touch screen (160x160 mono ranging up to 320x320 16K color) and a stylus, with no keyboard on most models. The keyboard lack was countered by a handwriting recognition system that took a very low-CPU power approach: Graffiti has its own gesture alphabet, which takes an hour or two to learn. By specifying the way every letter had to be drawn, and specifying where on the screen it would be recognized, a Motorola 68K was acceptable, and a 300MHz low-end ARM felt snappy.
PalmOS had a database format which lives on to this day as part of the Mobi ebook file spec.
It may have taken a bit longer, but I think the mainstream popularity of smartphones was inevitable.
I don't. I'm not a fan of Apple, but they really transformed our conception of a smartphone and made them a desirable device for the mass market. As I'm sure you know, early versions of Android looked nothing like it did once the iPhone was revealed. Even relatively modern smartphone platforms like WebOS completely failed to resonate with the public.