iPhones have gotten so good they can barely come up with anything new to add year-over-year. Young people in the west are almost exclusively buying iPhones because people like them.
They basically own the tablet space, if you're looking for a tablet there's almost no reason to go with anything other than an iPad. Same for smart watches, they make the best selling watching in the world.
For the amount of devices they move, they're shockingly reliable and have a smoother customer support / coverage system than any other company I've had to deal with. That's why people keep coming back.
It's pretty bizarre to say they're in decline. The only area I can see active decline is how badly they let Spotify eat their lunch with music streaming when they used to basically own digital music distribution.