Apple (AAPL 2.20%) investors have been fretting lately over where future iPhone growth will come from, as data continues to pile up that the high-end segment of subsidized, developed markets approaches maturity. Before even including the very distinct possibility of an affordable iPhone, Apple still has a powerful weapon in grabbing share from Google (GOOGL 1.68%) Android: customer loyalty.

That's the conclusion of a Yankee Group report, which expects Apple to top Android in the U.S. by 2015. The researcher surveyed 16,000 consumers over the past year, asking questions about smartphone ownership and purchasing plans. Current trends support the current duopoly structure of the market, with 84% of respondents planning on buying either an iPhone or Android in the next six months.

However, where the two dominant mobile platforms differ is loyalty.

Source: Yankee Group via AllThingsD.

That's a big difference in platform loyalty, and the effects in the long term will be a gradual shift toward iOS. There will always be defectors, but in this case the number of turncoats is extremely asymmetrical. Only 6% of iPhone users are looking to move to the other side, while 18% of Android users are eyeing iPhones.

Rival platforms from Microsoft and BlackBerry are hardly in the picture, with only 3% of iPhone users and 6% of Android users interested in abandoning the top two platforms.

By the time 2015 rolls around, Yankee Group is projecting iOS to surpass Android in U.S. market share. By 2017, Apple could be enjoying 42% of the domestic smartphone market, while Android sits idly by with a 34% slice. Yankee Group exec Carl Howe compares the platforms to leaking buckets of water, except that Google's leaks a lot more than Apple's.

Even as unit growth in the U.S. slows, Apple can still gain share through Android defectors.