Along the way consumers will start to tap out and leave Apple's ecosystem altogether.
Maybe that's already set in motion, people are keeping their phones for much longer so it will take a while for the effects to be noticeable.
What's tapping out is the pool of first time iPhone buyers, but fortunately for Apple, iPhones are very 'sticky'. Existing users are very highly likely to replace an iPhone with a new iPhone, and in fact the bulk of sales have been to previous iPhone owners for several years.
When people talk about iPhone sales slowing, they're taking about growth slowing. Actual sales are just levelling off. From here on out we'll see some ups and downs, sure. It looks like we're in for a period of economic risk and uncertainty too which isn't good for anyone, but I don't see any good reason to expect a sustained decline compared to the rest of the market.