A speculative rendering of an iPhone with a curved screen. @MesutGDesigns / Tech designs There are two ways to think about the launch of the new iPhone, expected in September:

It will be the biggest iPhone launch Apple has ever done, eclipsing iPhone 6, as Apple has a massive installed base of users waiting to upgrade to the first truly new iPhone since 2015. Some are calling this the iPhone "super-cycle."

Or — conversely — it will be a distinctly "meh" launch, as iPhone users hold onto their phones for longer and longer periods, and delay replacing them until prices fall in 2018.

The second scenario is very real for Apple, as two of its flagship products already suffer from a lack of upgrade sales: The iPad and the Mac laptops and desktops. The problem is that Apple's devices are almost too good: People don't replace their iPads because they don't need to. An old one works just as well as a new one for most users. iPad sales declined 19% last quarter as a result.

And Apple's newer Macs are fine. But I am typing this on an old Macbook built in 2012 — and it works just fine, too. I am under no pressure to get a new one. Mac sales were flat in the most recent period.

Now the same thing is happening to iPhone.

Time was, the useful life of a smartphone was about two years. That also roughly coincided with wireless subscriber contract lengths, prompting a giant wave of people to buy a new phone about once every two years. But Apple's phones are well-made, and now they last much longer.

Since iPhone 6, Apple hasn't given the iPhone a dramatic update. If you're still using an iPhone 6 you haven't missed much by not upgrading to iPhone 6s or iPhone 7.

That quality and longevity are coming back to haunt Apple, according to Citi analysts Jim Suva and Asiya Merchant. Apple's installed base of customers is getting larger, suggesting that when they upgrade sales of the new "iPhone 8" will be massive. But at the same time, replacement rates are stretching from two years to three years, lowering the percentage of existing customers that make new purchases:

So, even though the installed base of iPhone users will be 500 million people this year (up from 300 million in 2015), the number of potential upgraders will fall from 85% to 75%. Android is also getting better at holding its customers, so the percentage of new users who switch into iPhone will decline from 10% to 4% of new sales, the Citi team believes:

Essentially, Apple has trained its customers to make a choice:

The result is that while Apple will likely see sales growth from the new phone it nonetheless complicates matters if the company wants to put on an overwhelming show of strength for the iPhone's 10th anniversary this year.

Citi has a "buy" rating on AAPL and believes the company will still get "strong growth" from iPhone 8.