By Philip Elmer-DeWitt
July 18, 2013

FORTUNE — On Wednesday, Asymco‘s Horace Dediu posted the chart at right in response to a statement by UBS analyst John Hodulik  — reflecting conventional wisdom on Wall Street — that “everybody has got a smartphone.”

Using comScore data, Dediu was able to show

  • That 40% of U.S. mobile customers still use feature phones and represent an untapped market
  • That except for BlackBerry, churn between smart platforms is still negligible; it’s the non-smartphone makers who are shedding customers
  • That even at 60% penetration, the rate of smartphone adoption is not slowing in any perceptible way.

Moreover, since the rest of the world’s mobile users own fewer than half as many smartphones per capita, Dediu concludes that, as he puts it, “4 billion people are about to switch.”

That’s what Dediu had to say.

But the story doesn’t stop there. As often happens on Dediu’s Asymco blog, his well-informed (and enviably well-behaved) readers picked up the ball and ran with it.

I recommend the entire thread, but if you have time for only one comment, scroll down to the long post by “Chaka10.” Using data from a proprietary survey conducted by Consumer Intelligence Research Partners, he (or she) was able to tease out how many iPhone customers in each of the past 12 quarters were first-time smartphone buyers, how many were switching from another smartphone platform, and how many were loyal Apple (AAPL) customers coming back to buy another iPhone.

Chaka10 worked out what he called a “replacement demand ratio” and showed that it follows an observable pattern, peaking consistently — and strikingly — two years after the launch of a new iPhone.

The conclusion he reaches that is most relevant to Apple investors is that current Wall Street estimates for iPhone sales last quarter significantly underestimate the total, because they underestimate the impact of replacement demand.

He unpacks Wall Street’s unit sales estimates — which ranged in Fortune‘s survey from 23 million to 30 million — to show that they imply a replacement ratio of 69% to 88%. In contrast, Chakra10’s analysis –which allows for the replacement of a lot of iPhones that are two or more years old —  implies a year-over-year growth in replacement sales of 243%

In other words, Wall Street, having bought into the “everybody has got a smartphone”  meme, may be seriously underestimating how many iPhones Apple actually sold in Q3 2013.

We’ll find out if he’s right next Tuesday.

Asymco link: “Everybody has got a smartphone.”

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