To Apple enthusiasts, it's hard to imagine that someone might go more than a year or two without upgrading to the latest and greatest iPhone.

With new plans from AT&T, Verizon, and Apple itself that make it easy (if perhaps not cost-effective) to trade in your iPhone for a new model every year, many gadget lovers are well-equipped to always have the newest iPhone. But many, many people — a majority of iPhone owners, in fact — do not upgrade every year, or even every 15 months, according to data released by Apple CEO Tim Cook on this quarter's conference call.

In fact, only 40 percent of iPhone users who — deep breath for caveat — owned an iPhone the day before the iPhone 6 was released in September 2014, have upgraded to one of the new, larger screened phones: either the iPhone 6, 6 Plus, 6s, or 6s Plus. That means 3 in 5 iPhone owners in September 2014 have yet to upgrade to new phones.

And it's been a very slow climb. A year ago, after the first quarter of iPhone 6 availability, barely 15 percent of iPhone owners had upgraded, and that's in a quarter where Apple sold nearly 75 million iPhones. See this chart:

So, if you read an article that says Apple's sales have flatlined (Apple sold roughly 75 million iPhones in both the December 2014 and 2015 quarters), just remember: there are millions of people still using the iPhone 4 or 5 who might be upgrading to Cupertino's newest phone in the next year or two. And that's a lot of phones.