Fewer than expected iPhone sales in Apple's (AAPL 0.01%) first quarter spooked investors, sparking an 8% sell-off. While shares have mostly recovered since then, the sell-off shows how sensitive the market is to Apple's growth story. The iPhone, which accounts for more than half of the company's revenue and an even larger portion of its operating profits, isn't contributing the same monstrous growth to Apple's top line as it used to. Units were up just 7% from the year-ago quarter. Single-digit growth like this has investors wondering how much the Cupertino tech giant's growth can decelerate.

iPhone 5c.

Saturation? Not yet.
While growth in Apple's iPhone sales may, indeed, decelerate further, there is little chance of year-over-year declines in the near future. We can thank one statistic for this hopeful outlook: Nearly half of the globe opted not to buy a smartphone in 2013, instead choosing a traditional feature phone. Specifically, 46.5% of global handset shipments in 2013 were feature phones, according to Gartner. While Apple probably won't be benefiting from this market opportunity very meaningfully in the short term, it does suggest that the smartphone market isn't saturated yet. Over the long haul, this market presents Apple opportunity for further growth.

Indeed, Asymco Apple analyst Horace Dediu proposes that talk of saturation is largely related to a point of inflection in growth rates in North America and Western Europe. These markets account for only 11% of the world's population. For the rest of the world, saturation is much further off.

Or here's another way to look at Apple's opportunity. Zooming way out, Apple smartphones account for 8.3% of the total 2013 mobile phone shipments, up from 7.5% last year.

Gartner analyst Anshul Gupta expects Apple's share expansion to continue in the first quarter of 2014, benefiting from the recent NTT DoCoMo and China Mobile arrangements.

With Apple adding NTT DOCOMO in Japan for the first time in September 2013 and signing a deal with China Mobile during the quarter, we are already seeing an increased growth in the Japanese market and we should see the impact of the last deal in the first quarter of 2014.

The deal with China Mobile should especially be a contributing factor to Apple's smartphone sales in 2014. Not only is the carrier the world's largest with 767 million wireless subscribers, but it also positions Apple strategically in a very fast growing market. Smartphone sales in China grew by 86.3% in 2013 from the year before, according to Gartner.

New categories are a bonus
If Apple's smartphone sales can continue to grow at single-digit rates, Apple's plans for "new categories" in 2014 is just a bonus at today's valuation. It's this low downside risk and meaningful upside opportunity that gets me excited about Apple stock.