If you focus on the long term, as I do, you might have been greatly rewarded by an investment in Apple (AAPL) stock through the years. For example, over the past decade the stock is up nearly ten-fold.

Much of that success was due to the iPhone. So far, the Cupertino, CA-based company has shipped over 800 million units and this year another 230 million or so will probably fly off the shelves. About two-thirds of total corporate revenue, and the bulk of net income, can be traced to the iPhone.

Add in shipments of iPods and iPads, and more than a billion smart mobile devices have been sold. Throw in a few hundred million Mac computers and…well you get the picture.

But that was the past. Paraphrasing an old adage: “What will you do for me down the road?”

Let’s examine the main threats to AAPL stock’s long term growth.

No. 1: Innovation

The real reason behind AAPL’s phenomenal performance is its corporate culture, the “Think Different” strategy that has driven design, engineering, manufacturing, and marketing operations at the company over the years. You can read more about it in the book, “Insanely Simple: The Obsession That Drives Apple’s Success”, by Ken Segall.

This approach has led to periods of fantastic innovation (not to be confused with invention although that is important too) and to a succession of wildly popular consumer products that build off each other and reside in the same ecosystem. The iPod led to the iPhone which led to the iPad and Apple TV and just recently to the Apple Watch. Complementing the hardware is the iOS mobile operating system and related services such as iTunes, the App Store, Apple Pay, Car Play, and Apple Music. What’s next? Rumors have been flying around Silicon Valley that the company is developing an electric vehicle.

Innovation is not necessarily being the first to market but it means being the best to market. If the innovation stream dries up, and that is what many investors have been worried about lately, it could mean a problem for AAPL stock. Then it is possible that the consistent, double-digit growth experienced for such a long time could slow down. Therefore, one of the biggest threats to Apple, is the Apple culture itself.

The success of Apple Watch could be an interim metric for the path of innovation. Investors are sure to be scrutinizing financial statements and other information for any signs of problems in this area.

Other measures could be a change in certain iPhone specs. Over the years AAPL has made a point of charting CPU speed and it has reported an exponential increase with each successive model release. The latest, iPhone 6s, is nearly 30% faster than the previous version.

Finally, looking further down the road the “Apple Car” program could also be a proxy to gauge innovation.

No 2: Market

AAPL dominates the smartphone market. Although it ranks further down the list in number of units sold, it is by far No. 1 in net income with 94% of the industry total. Only one other manufacturer, Samsung Electronics (SSNLF), makes a profit. The hundreds of others that compete in the market lose money.

If the global market for high-end smartphones plateaus, as is already the case for North America and Europe,  it could be a problem for Apple’s main cash cow. An important area for Apple is China, which is experiencing double-digit growth for now.

AAPL has been taking steps to diversify its lineup and now has five major product/service segments that each generate over $10 billion in revenue. All but the iPad are growing and that is probably due to the fact that the entire market for tablets is slowing down in the face of changing user preferences. Sales of smartphones with bigger displays could be eating into those of smaller sized tablets. Apple’s answer to this was releasing the larger iPad Pro last year.

Issues in any of the four non-iPhone categories could also spell trouble over the long-term. In addition to be being a measure of continuing innovation, the success of Apple Watch is important from a market perspective too. So far indications are that it has vaulted to the top of the industry.

No. 3 Increasing Debt

A few years ago Apple, prodded by high-profile activist investors such as Carl Icahn, decided to implement a shareholder capital return program. In a series of moves the board of directors eventually authorized up to $200 billion for dividend payments and AAPL stock buybacks. As of September 2015 $140 billion has been spent.

The company took on a significant amount of debt (at the end of the 2015 fiscal year the long term debt/shareholder equity ratio was 0.54) as part of this campaign. As recently as 2013 Apple had no long term debt on its books.

While it isn’t a big issue for now it is something to keep an eye out over the long term. Any reauthorization above the current $200 billion capital return program limit should at least raise a yellow flag.

Conclusion

As I see it there are three primary threats to the long-term growth of AAPL stock. Slowing innovation, changes in the markets for its popular products and services, and debt issues could all be catalysts for a reversal of fortune. Investors would be wise to keep an eye out for any movement in any of these areas.

Disclaimer: The author owns shares of Apple, Inc.