One of the more interesting parts of Apple's WWDC keynote this year was the announcement that every device that currently runs iOS 8 can be upgraded to iOS 9. This is a big deal for two reasons.
Every prior major release of iOS has obsoleted one generation of hardware, and generally made the earliest still-supported hardware for that release feel slower. With iOS 9, it seems Apple is taking a new approach by building iOS 9 up in an additive fashion, making sure they don't overstep the boundaries afforded by their target hardware, and stopping before any significant performance degradation is incurred. Some early reports are even saying their older devices are running faster with the first iOS 9 betas, compared to iOS 8, which is unheard of! This new approach suggests Apple is ready and willing to change the "planned obsolescence" narrative that has become the norm around their annual software/hardware release cycles.
At a higher level though, the fact that Apple are able to deliver another year of support, illustrates the tipping point that we've reached with smartphones. No longer do we have to buy a new device and 18 months later worry about whether the manufacturer is going to support it with the latest updates, and whether we'll be able to download the newest apps and run them. By the time iOS 10 comes out in 2016, the iPhone 4S will be nearly 5 years old, running the latest OS. Five years. The original, first generation iPhone will be 9 years old at that point. The smartphone industry evolved from "here's this revolutionary new kind of thing, a full computer in your pocket" to "here's a device that will be good enough for you to keep for 5-6 years" in 4 years.
There are caveats - new hardware features can tempt many people into making an upgrade, whether that's a new design, a better camera, the TouchID fingerprint sensor, or just the ability to play newer, more demanding games with a faster processor and GPU. But the fact that your next smartphone is likely to remain current for 4-5 years means that the industry should look for the bulk of further innovation in the next decade to come elsewhere. There will be new apps and services that run on our devices, as technology disrupts industries that are still in an early stage of penetration (for example, property, healthcare, education). There'll also be entirely new categories of hardware looking to earn mass market status - self driving cars, VR and smartwatches are an obvious three. In the here and now though, the smartphone, by becoming good enough, is approaching its zenith.