Features like Siri?
I only ever saw/heard people use Siri the first time they got an iPhone or on comedy shows. In the last few years I haven't ever seen or heard of anyone using Siri. I have known of people using voice control*, like phone dialling from the car or from their headset while walking but never bothering with Siri.
If that's an example of the amazing new features (which are several years old) that are going to get people buying new or second iPhones, then the market has matured and sales will slow down. There's been nothing new of any real significance for several generations, in any phone, just slow iteration. There's little need to upgrade other than because the phone is broken, slowed down by OS updates or now a bit too small. With Apple prices people aren't keen to splash cash every couple of years unless necessary and it is becoming less necessary each year - no matter how much Apple tell us "it's a game changing changer change".
This doesn't mean that smartphones or Apple are dead, but just like TVs we buy them to last 5+ years not until next year. There's nothing wrong with that but electronics firms have got used to massive sales and haven't quite had the guts to tell their investors that it won't last for ever.
* - Siri and voice control on iPhones aren't the same, the phone's commands "dial mum" can be driven by voice with Siri turned off, it was present before 'Siri' was even a thing