Apple (NASDAQ:AAPL) has seen tremendous demand for its new iPhone 6 and iPhone 6 Plus models since they were released in September. Inventory remains tight two and a half months after the new devices went on sale, especially for models with higher storage capacity.

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Inventory of Apple's new iPhones remains tight. Photo: Apple.

Despite this strong demand, a few Wall Street analysts have speculated that iPhone sales growth could soon come to a sharp halt. These analysts suspect sales of the new iPhones are being driven by replacement demand from current iPhone users being "pulled forward" from future quarters.

However, an analysis of who has been buying the new iPhones demonstrates this worry is overblown. In fact, while there has been healthy iPhone replacement demand, it has mainly come from users who delayed their upgrade to wait for the iPhone 6 and iPhone 6 Plus.

The iPhone downside scenario
Even the most bearish analysts -- like Adnaan Ahmad of Berenberg Bank, who has an absurdly low $60 price target for Apple stock -- admit there has been great demand for the new iPhones.

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Even bearish analysts admit that demand for the new iPhones is extraordinary. Photo: Apple.

Pacific Crest Securities analyst Andy Hargreaves isn't quite as negative as Ahmad on this stock, but he does think Apple shares are worth about $96 -- significantly less than the recent trading price. Last week, Hargreaves reiterated his recommendation that investors sell some of their Apple shares.

Like other bears, Hargreaves recognizes that iPhone sales have been buzzing along. "However, we believe extraordinary replacement activity, rather than stronger-than-expected new iPhone user growth, is driving much of the strength," he wrote. This could be pulling demand forward from future quarters, leading to a big letdown in 2015.

Upgrades vs. new users
To evaluate Hargreaves' argument, let's look at who has been buying the iPhone 6 and 6 Plus. Third-party research firms such as Consumer Intelligence Research Partners and Kantar Worldpanel have found that 80% or more of the new iPhones have been sold to people who already owned iPhones.

It's not surprising that iPhone users bought up most available models of the new devices, given the severity of supply constraints. Die-hard Apple fans were presumably more likely to wait in line for a new iPhone or order it sight unseen.

Non-iPhone users might be waiting for better retail availability so they can compare the iPhone 6 and iPhone 6 Plus to each other and to the top Android smartphones. Even so, if close to 20% of the new iPhones have been sold to new users, then fresh-to-iPhone customers have snapped up about 10 million iPhones in less than three months. That's a meaningful number.

A look at upgrade demand
Upgrade demand has been the biggest sales driver for the iPhone 6 and iPhone 6 Plus. However, a look at changes in iPhone usage patterns (tracked by Fiksu) shows iPhone replacement demand hasn't been "pulled forward" in a meaningful way.

The following table compares usage of various iPhone models in the first two weeks of September (just before the iPhone 6 and iPhone 6 Plus went on sale) to usage in the first few days of December.

Model

Usage 9/1-9/14

Usage 12/1-12/3

iPhone 6/iPhone 6 Plus

0%

15%

iPhone 5s

27.9%

27.6%

iPhone 5c

9.5%

8%

iPhone 5

25.7%

21.1%

iPhone 4S

22.6%

16.8%

Older iPhones

14.4%

11.4%

Source: Fiksu iPhone Usage Monitor (totals may not sum to 100% due to rounding).

As the iPhone 6 and iPhone 6 Plus have surged from zero to a 15% share of all iPhone usage, the change in usage patterns for other models says a lot about who is upgrading to the new devices

If demand were being pulled forward, you would expect to see many early upgrades from people who bought iPhones last year (when the 5s was the top-of-the-line model). Based on a typical two-year contract cycle, these people would not have been eligible to upgrade for another year. The introduction of larger screen sizes and Apple Pay could convince many loyal Apple fans to switch early, though.

By and large, this isn't what's happening. Since the beginning of September, iPhone 5s usage has dropped only from 27.9% to 27.6% of all iPhone usage, according to Fiksu. While there is undoubtedly some margin of error for Fiksu's estimates, this is not a significant change.

To be fair, there is plenty of "noise" in these numbers. On the one hand, the iPhone user base has probably expanded since September. (As noted above, about 10 million new users might have bought iPhones in the last few months.)

On the other hand, the iPhone 5s is still selling in decent numbers, so some iPhone 5s users who have upgraded to the 6 or 6 Plus have been "replaced" by new iPhone 5s users. Additionally, people who upgraded from the iPhone 5s might have sold or given away their old device, keeping it in the active usage base.

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Upgrades from last year's iPhones haven't been the main driver of iPhone 6/iPhone 6 Plus sales. Photo: Apple.

It's debatable how many people have upgraded from the iPhone 5s in the past few months, but it's crystal clear that people have been upgrading from older iPhone models in droves.

Since the beginning of September, usage of the iPhone 4S has plummeted by nearly 6 percentage points (from 22.6% to 16.8%). Additionally, usage of the iPhone 5 dropped from 25.7% to 21.1% and usage of the iPhone 4 and earlier models fell from 14.4% to 11.4%. The sudden drop in usage share for these models can only be explained by mass upgrades to the new iPhones.

Comparing sources of iPhone 6 and iPhone 6 Plus sales
Thus, the lion's share of upgrades to the iPhone 6 and iPhone 6 Plus have come from people who were previously using the iPhone 4S (initially released in October 2011) and older models.

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Upgrades from the iPhone 4S and iPhone 5 have been the two biggest sources of iPhone 6 and iPhone 6 Plus sales. Photo: Apple.

This isn't especially surprising. Rumors about larger-screened iPhones were circulating widely for more than a year before the iPhone 6 and iPhone 6 Plus debuted. Many people who bought the iPhone 4S in late 2011 or 2012 on a two-year contract might have decided to wait for the new iPhones after becoming upgrade-eligible within the past year.

The second-largest source of upgrades has been people previously using the iPhone 5. Many of these people (especially in the U.S.) could be on standard subsidy plans and just became upgrade-eligible after buying the iPhone 5 in fall 2012.

Early upgrades from the iPhone 5s (or iPhone 5c) account for a much smaller share of iPhone 6 and iPhone 6 Plus sales, based on Fiksu's data. This category is probably on par with sales to new iPhone buyers in magnitude.

The key takeaway: iPhone sales are not being pulled forward from future quarters in a meaningful way. Instead, customers willing to wait for Apple's bigger handsets pushed back iPhone upgrades that might have occurred earlier in 2014.

This means iPhone demand should remain strong throughout 2015 and beyond. The vast majority of the more than 100 million iPhone 5s/iPhone 5c users are happy to wait to upgrade to big-screened iPhones.

That lingering upgrade demand, along with steady growth in the user base from Android-switchers and people getting their first smartphones, will keep iPhone sales humming along for the foreseeable future.

Adam Levine-Weinberg is long January 2016 $80 calls on Apple and short January 2016 $120 calls on Apple. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.