I was not at all surprised to read that the smartphone era has hit a slowdown.
What's causing this are a few factors. First smartphones are increasing in price at the top end. When an iPhone is over $1000 it's becoming a status symbol as much as a communications appliance. Even rival Samsung is at that point of pricing themselves to where the average buyer has to think about the investment.
The arrival of dual SIM devices is also triggering a slowdown. Now instead of two phones, people only need one with two service providers or plans. eSIMs will only further trigger less need for a second phone.
Lastly is the hype of 5G. It's enough to cause people to say, "I'll wait to upgrade to a device that will work on 5G vs. having to buy another phone next year."