Same thing that happened to PCs. People are finding that their 3-5 (or more) year old devices are still perfectly adequate for their needs.
My mom still uses my 2006 Mac Mini Core Duo running Windows 7 when she has students over when she tutors.
Her main computer is a 2007 era Sony Viao Core Duo with 2GB of RAM running Windows 7 and Office.
I'm using a 2008 Core 2 Duo 2.66Ghz Core 2 Duo with 4Gb of RAM as a Plex Server.
I just replaced a Pentium Dusl Core laptop with 4GB of RAM circa 2009 with a newer laptop. I still use it occasionally. It's my only computer with a DVD writer.
It helps that my main use of a computer involves work and my main laptop is work provided.
What happened to PCs was market saturation followed by people choosing smartphones over PCs.
It is inevitable that smartphones sales will stall and the growth will decline because the smartphone market is saturated.