In my opinion, this is not "peak-smartphone" in the way it is "peak-PC". PC growth slowed because people were buying/using Smartphones instead.
Smartphone growth (especially iPhone growth) has slowed because the high penetration rate among those who could afford a smartphone. However, nothing is replacing smartphones at the moment.
Every time there is a rush to the good ol' terminal - big computer days people remember that we aren't connected 24/7 and cloud applications work best in the conditions where having a big box under your desk isn't an inconvenience.
Considering that "smartphones" (or any other similar device) is used often when there is no effective connectivity, and even when there is one you usually pay a higher penalty for using that connectivity as far as your battery life goes than using the onboard processing power - this is because the presentation layer costs just the same, your hardware power consumption (screen, sensors, audio, memory etc.) are almost the same, and the CPU is often more power efficient than your radio even at pretty intensive tasks.
Could it be wearables?
What the big dogs (AMZN/GOOG/MSFT) seem to be pushing for consumers now isn't wearables as much as voice accessed AI assistants like Alexa.