This last quarter they sold nearly 50 million iPhones with 30% y/y growth. They did another 20 something million iPads for nearly 50% y/y growth. As that slows they're still going to get a lot of altitude before they see declines in sales numbers, if they ever see declines in sales numbers.
If there is ever a decline, it would start with hundreds of billions in the bank and hundreds of millions of unit sales per year. If there ever is a decline, you are right, it would take a long time...a VERY long time.
They would have to pull a metro to F this all up or someone else invent a phone that gives awesome BJs. A 5" screen and NFC ain't gonna do it.