Credit Suisse has a great piece of data showing why iPhone sales may already be in decline

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Apple CEO Tim Cook.

If Apple did sell fewer iPhones this quarter than the year before, it will be a dramatic, historic occasion. For years, Apple CEO Tim Cook has introduced his earnings calls by proudly announcing that, once again, it was iPhone's "best" quarter ever, or that a new sales "record" was broken.

The prospect of him being unable to do that - the very idea that Apple might sell fewer phones - sounds like something out of the Twilight Zone.

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This morning Business Insider obtained a note to investors from Credit Suisse that goes a long way to explaining why iPhone sales may be in decline now.

This year, for the first time ever, the net number of new smartphone customers will go into decline. The industry is moving closer to 100% penetration of potential smartphone owners, and in future most sales will be upgrades or new phones for existing customers, not new customers buying their first phone.

That fact explains why Apple CEO Tim Cook has become obsessed with "Android switchers" - people who already own smartphones but change them for iPhones - on his recent earnings calls. And it explains why Apple's big new marketing push is to persuade customers to sign up for pay-as-you-go plans that will upgrade their phones to the new iPhone model every year. Apple is running out of new people to sell phones to, and the quality of iPhone is so good that users don't need to upgrade them every year, because they last a long time.

Here's Credit Suisse analyst Kulbinder Garcha and his team:

Smartphone penetration reaching 100%. Given an installed base of 3.3bn smartphone subscribers we estimate at the end of 2015 and our long-term smartphone addressable market forecast of around 5.8bn, the effective penetration at the end of 2015 is becoming significant, at 57% by our estimates. Based on penetration gains driven by improving affordability, we believe that smartphone adoption will gradually rise to 100% over the next 5 years.

And here is their terrifically useful chart:

Credit Suisse

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