The people who bought in 2019 probably aren't going to buy again in 2020, you're right. It will be the people who last bought in 2018, or 2017, or even earlier who will.
Apple reported that their "installed base" went from 1.4 billion to 1.5 billion over the past 12 months, that's the number that really matters to them not whether they see increases in year over year sales. They can keep growing the installed base even if yearly sales go into decline since every new iPhone sold means an old iPhone traded in / sold to a second hand owner, and many of those are selling on to a third hand owner. A bigger installed base means more app sales, more potential customers for services like Apple Music, and perhaps some future owners of other Apple devices like Macs or Watches.
Biting the hand that feeds IT
