There were already plenty of reports indicating the iPhone 7 would feature only modest improvements relative to the iPhone 6S, and thus represent a break from Apple's (AAPL) - Get Report historical approach of delivering a major iPhone overhaul every two years, and smaller improvements in the year in between.

Now, a Pacific Crest report suggests Apple doesn't expect the device to drive massive iPhone upgrades the way the iPhone 6 did.

Pac Crest's Mike McConnell states "indications from the supply chain" point to 72 million-76 million iPhone 7 units being produced in the second half of 2016, down 15%-20% from the number of iPhone 6S units produced a year earlier. This leads him to forecast iPhone chip shipments will peak in September rather than in the fourth quarter (as was the case in prior years).

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One silver lining: McConnell reports iPhone SE sales have been better than expected, and forecasts 2016 shipments of the 4" phone will hit 30 million. Nonetheless, low iPhone 7 expectations have led him to downgrade audio codec chip supplier Cirrus Logic (CRUS) - Get Reportand RF chip supplier Skyworks (SWKS) - Get Report have been downgraded, even though both are expected to see their chip content rise with the iPhone 7 relative to the 6S. Cirrus, Skyworks and other iPhone suppliers sold off in response.

McConnell's note isn't the only one out today that raises concerns about sales of the iPhone, which accounts for well over half of Apple's revenue and gross profit. Citi's Jim Suva cut his calendar second and third quarter Apple estimates, citing soft demand ahead of the iPhone 7's expected fall launch, Brexit-related macro uncertainty and -- perhaps most notably -- lengthening smartphone replacement cycles. In April, his firm estimated the average phone-replacement cycle will grow to 29 months in the first half of 2016 from 24-26 months during the prior two years.

Suva is far from the only person to point out smartphone upgrade cycles are stretching out. A recent survey of U.S. smartphone users by ad tech firm Fluent found 42% said they take three years or longer to upgrade their phones. Thirty percent said they upgrade every two years, and 28% said they take less than two years. Chalk this trend up to both the decline of carrier phone subsidies in favor of unlocked phone purchases and monthly installment plans, and a slowing pace to hardware innovation.

This slowing upgrade pace provides some context for an interesting note from Cowen's Tim Arcuri. Thanks to strong iPhone 6 sales in 2014 and 2015, Arcuri expects the percentage of the iPhone installed base owning a phone that's more than two years old to jump to 43% from a current 33% over the next five quarters. That, in turn, leads him to speculate Apple is holding off on delivering major iPhone hardware improvements until 2017, when its base would be "incredibly ripe" for upgrades.

Granted, if next year's iPhones deliver much bigger innovations than this year's models, it probably has as much (if not more) to do with the readiness of those innovations as upgrade cycle dynamics. For example, if Apple wanted to have the iPhone 7 use edge-to-edge OLED displays -- 2017 iPhones are rumored to feature them -- it would likely struggle to secure adequate supplies.

It's also quite possible that some of the other technologies rumored to arrive next year, such as over-the-air wireless charging, more advanced haptic feedback and a fingerprint sensor embedded into a phone's touchscreen, aren't ready yet.

Still, with upgrade cycles lengthening, there is some logic to stretching out the interval between major iPhone revamps. Even if that means 2016's iPhone launches won't provide major relief for what has thus far been a pretty rough year for iPhone sales.