So everyone has got a phone then. They had a good run.
This should be good news. Less people have to slave away making iPhones because everyone has got one and less people have to slave away paying for getting themselves a phone that does everything they wish for in a phone. Yet with the wonders of capitalist growth this is met by the people that write for the media as a concern. Every curve has to be exponential.
Phones have to be replaced at some point. I don't think the number of phones sold in general is going to start going down. Population is increasing, more people are coming online, and phones seem to last as long as they always have (~2 years in my experience)
We saw this before with personal computers. So I don't know what you can't see with the phone situation. PCs are now a commodity item that has razor thin margins and any from the last decade are fine for everyone except for developers and gamers. Phones are now the same, everyone has got one and they get replaced when people drop them badly, misplace them, get them stolen or if they come off a contract. But that contract model is dying as people don't think that it is worth buying them in the way people paid for TVs in the 1970's by instalments.
Batteries last for about 2 years, top of the line phones can easily last 4. They are after all made of glass, steel, and have plenty of ram.
Take the Pixel XL from Oct 2016: 5.5 inch screen, 2560x1440, 4GB ram, quad core CPU, 128GB of storage. The pixel 3 XL isn't much different (same ram, same storage, 10nm CPU instead of 14nm).
If apple keeps offering the $29 battery replacement I'd expect the average iphone owner will keep them for significantly longer. They already claim about $9B of losses because people are replacing batteries instead of buying $1k phone replacements.
