Do you think iPhones are too expensive and that's why people are not buying? That's not the problem why people are not upgrading to new iPhones
The main reason why Apple reduced its revenue forecast was because of lower sales in China because of the trade war. But Tim Cook did mention that there are less iPhone upgrades.
Even Samsung has this problem and they recently made there are revenue guidance stating that profits will be down by 13%
Is the smartphone business saturated?
There are currently 3 billions smartphone users in the world and this number is increasing but most new smartphone users don't actually buy high end smartphones such as the iPhone.
Apple actually makes money from existing users who are upgrading every year. But lately there has been seldom any innovation in the smartphone. New features are being added but no big innovation. That's the main reason why people don't want to buy a new smartphone.
The next big innovation will be with 5G smartphones this year for Samsung and next year for Apple. Apple is waiting for fall of 2020 to launch the new 5G iPhones because most of the carriers have not installed 5G networks yet.