Horace Dediu looks at the convergence of the US and EU5 smartphone markets and predicts we will reach saturation in about 2 years. Saturation here means that nearly (but not 100%) of adults who purchase their phone (eg not kids, and not employer-provided phones) will have some sort of smartphone.
We can draw a couple of quick conclusions on this. First, the biggest growth spurts are probably going to slow (again, US and EU5 – China et al will still be huge and growing fast). Second, users will be harder to come by since they must be poached from another platform rather than moved off of a feature phone. This means that the next two years are pretty critical for platform growth and that the platforms had better be ready to provide meaningful reasons for switching and protecting reasons for staying.