Apple's iPhone 6 led the company to tremendous gains -- but is the iPhone-growth story over? Source: Flickr user Karlis Dambrans.

As the world's largest publicly traded company, Apple (AAPL 0.06%) merits an opinion from nearly every market participant on its path forward. And while these opinions diverge wildly, with bulls and bears vehemently disagreeing, the argument generally boils down to sales of one product: Apple's iPhone line.

And that makes sense. After Apple introduced the product in 2007, the iPhone has been an integral part of the company's revenue haul. And it appears to be increasing in relevance. For the past two fiscal years, Apple's iPhone has gone from 56% of its revenue haul to 66% on the back of tremendous growth from the product amid struggling performance from the rest of the company. Here's a visual of the iPhone's revenue over the past two years versus the rest of Apple on a quarterly basis:

Source: Apple's 10Qs. Revenue figures in billions.

The company is becoming increasingly dependent on the iPhone for top-line performance. And if the newest report from Morgan Stanley analyst Katy Huberty is correct, it appears Apple will watch its iPhone revenue drop next fiscal year.

More analysts are pointing to lower iPhone sales
And it isn't as if Huberty is an unreasonable Apple bear. As one of the most visible Apple analysts, Huberty is conservative but generally optimistic. As of this note, Apple is still listed as a "Best Idea" stock purchase with Morgan Stanley, although Huberty did lower the price target 12% -- from $162 to $143 – as a result of her newest research.

As for actual specifics, Huberty now estimates Apple iPhone sales to drop to 218 million units, down 6% from 231.2 million in the recently completed fiscal year. If we assume the ASP per unit will remain the same, a 6% revenue drop from Apple's iPhone will require top-line growth of 11.2% from the rest of the company to prevent a full-company revenue decrease. Morgan Stanley's price-target decrease suggests the company doesn't expect that to happen.

And it's not just Morgan Stanley and Katy Huberty: A growing host of analysts are reporting Apple to report lower iPhone unit sales next year, as a combination of a slowing market overall and last year's tremendous results have raised the bar to atmospheric levels. Credit Suisse cut its estimates for next year's iPhone sales from 242 million to 222 million. In addition, Pacific Crest and Ming-Chi Kuo have mentioned year-on-year declines.

Last year's performance will be tough to repeat, but does Apple have a trick up its sleeve?
One thing's for sure: Apple has an extremely high bar set for it going forward. On a units-sold basis, the company moved 37% more iPhones during the past fiscal year as the iPhone 6 and iPhone 6 Plus proved extremely popular. In the thankless world of Wall Street, however, overperformance tends to get rewarded with increased expectations. Here's a look into Apple's iPhone unit sales:

Source: Business Insider/Statista. Unit figures are in millions. Sales estimates for 2016 from Morgan Stanley.

The last time the company attained a level of growth higher than it did last year was fiscal 2012, a year that benefited by a later-than-expected iPhone 4s launch, which occurred in the first fiscal quarter of 2012 rather than in late 2011. The end result was that the company had two phone launches in one fiscal year, making the 73% growth more of a timing issue than anything else.

That said, it was interesting that Morgan Stanley's analysis didn't include Apple's rumored 4-inch iPhone. While the rumor mill has swirled in regard to a spring release, Huberty doesn't seem to incorporate this into her analysis. And while we're unsure if this will introduce new users into Apple's ecosystem or merely cannibalize higher-priced iPhones, it's a material change from current models. More broadly, though, Apple's stock has been beaten down as a result of increasing bearish sentiment. If Apple is able to outperform, long-term investors could be rewarded by buying a high-performing stock at a discount.