Apple on Wednesday warned investors that its revenue for the last three months of 2018 would not live up to previous estimates, or even come particularly close. The main culprit appears to be China, where the trade war and a broader economic slowdown contributed to plummeting iPhone sales. But CEO Tim Cook’s letter to investors pointed to a secondary thread as well, one that Apple customers, environmentalists, and even the company itself should view not as a liability but an asset: People are holding onto their iPhones longer.

That’s not just in China. Cook noted that iPhone upgrades were “not as strong as we thought they would be” in developed markets as well, citing “macroeconomic conditions,” a shift in how carriers price smartphones, a strong US dollar, and temporarily discounted battery replacements. He neglected to mention the simple fact that an iPhone can perform capably for years—and consumers are finally getting wise.

As recently as 2015, smartphone users on average upgraded their phone roughly every 24 months, says Cliff Maldonado, founder of BayStreet Research, which tracks the mobile industry. As of the fourth quarter of last year, that had jumped to at least 35 months. “You’re looking at people holding onto their devices an extra year,” Maldonado says. “It’s been considerable.”

A few factors contribute to the trend, chief among them the shift from buying phones on a two-year contract—heavily subsidized by the carriers—to installment plans in which the customer pays full freight. T-Mobile introduced the practice in the US in 2014, and by 2015 it had become the norm. The full effects, though, have only kicked in more recently. People still generally pay for their smartphone over two years; once they’re paid off, though, their monthly bill suddenly drops by, say, $25.

The shift has also caused a sharp drop-off in carrier incentives. They turn out not to be worth it. “They’re actually encouraging that dynamic of holding your smartphone longer. It’s in their best interest,” Maldonado says. “It actually costs them to get you into a new phone, to do those promotions, to run the transaction and put it on their books and finance it.”

Bottom line: If your service is reliable and your iPhone still works fine, why go through the hassle?

“There’s not as many subsidies as there used to be from a carrier point of view,” Cook told CNBC Wednesday. “And where that didn’t all happen yesterday, if you’ve been out of the market for two or three years and you come back, it looks like that to you.”

Meanwhile, older iPhones work better, for longer, thanks to Apple itself. When Apple vice president Craig Federighi introduced iOS 12 in June at Apple’s Worldwide Developers Conference, he emphasized how much it improved the performance of older devices. Among the numbers he cited: The 2014 iPhone 6 Plus opens apps 40 percent faster with iOS 12 than it had with iOS 11, and its keyboard appears up to 50 percent faster than before. And while Apple’s battery scandal of a year ago was a black mark for the company, it at least reminded Apple owners that they didn’t necessarily need a new iPhone. Eligible iPhone owners found that a $29 battery replacement—it normally costs $79—made their iPhone 6 feel something close to new.

“There definitely has been a major shift in customer perception, after all the controversy,” says Kyle Wiens, founder of online repair community iFixit. “What it really did more than anything else was remind you that the battery on your phone really can be replaced. Apple successfully brainwashing the public into thinking the battery was something they never needed to think about led people to prematurely buy these devices.”