Apple (NASDAQ:AAPL) has been extraordinarily successful in generating revenue growth out of stagnating unit sales over the past year, primarily thanks to the widely discussed price increases that the company has been implementing ever since introducing the iPhone X in 2017, which started at $999. The company's ability to grow overall iPhone revenue should be what matters most. Apple certainly agrees, which is why it will no longer report unit sales and will only disclose iPhone revenue going forward.

Still, investors continue to fret over the prospect of weak unit sales, even if they will no longer be privy to official unit sales data. As iPhone suppliers continue to post soft results -- including lens supplier Largan Precision today -- Apple investors can't help but worry about unit volumes.

Chart comparing iPhone revenue growth to unit growth

Data source: SEC filings. Chart by author.

Price increases compound the initial problem of longer upgrade cycles

Decelerating unit volumes continue to weigh on analyst sentiment. For example, this week New Street Research analyst Pierre Ferragu worried that "all [average selling price] gains of recent years could be lost in unit decline in the coming years." Apple has ramped up promotional activity in recent weeks, which Ferragu sees as evidence of weak demand. CFRA analyst Angelo Zino wrote in a research note this morning that the company "continues to try to push toward these higher-price devices, and we think that is continuing to cause a slowdown in the replacement cycle."

Speaking of average selling price (ASP) gains, here's what Ferragu is referring to:

Chart showing iPhone ASPs

Data source: SEC filings. Chart by author.

Smartphone upgrade cycles have been steadily getting longer over time. In 2012, the average upgrade cycle was 22 months, according to Recon Analytics. It reached 28 months near the tail end of 2015, hitting 29 months for the first half of 2016, according to Citigroup. By the end of 2017, we're talking about 32 months, according to NPD. Beyond third-party estimates, even carriers are now exploring 36-month installment plans, arguably the most direct evidence, as carriers have firsthand data on upgrade cycles.

iPhone XS Max lineup

iPhone XS Max can cost upwards of $1,450. Image source: Apple.

Therein lies the great irony. My view has been that iPhone price increases are Apple's strategy of coping with longer upgrade cycles. Consider a customer that buys a $1,000 phone (like a base iPhone XS) and keeps it for two years compared to a customer that buys a $1,500 phone (like a fully loaded iPhone XS Max) and keeps it for three years. The annual revenue that Apple gets from that customer is the same. But by adopting this strategy of price increases as a way to offset longer upgrade cycles, the tech titan inadvertently encourages customers to keep those phones longer, too, further compounding the problem it was trying to initially address. The market will be at three-year upgrade cycles in no time.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.