To give full growth to that which still doth grow?
I'm going to try to observe the evolution of the size of customer installed bases, which should gradually expand (or contract) in typical s-shaped patterns (aka sigmoid, logistic or other similar functions) when plotted over time. This derives from the notion that significant, long-term changes in customer adoption driven by innovative technologies require time to play out, as different segments of customers react differently to change, even assuming immediate information transfer about the innovation (which is not the case even in the internet age).
A constant 5-year life cycle is assumed for these traditional PCs over all the historical data. This greatly simplifies the computation of the total size of the installed base at any given point in time as the the sum of all the unit sales over a trailing 5-year moving window. Similarly, a standard 2-year life cycle is assumed for mobile PCs. iPods and iPads (and other tablets, PDAs, etc) perhaps have longer life cycles and iPhones (and phones in general) probably have slightly shorter average times before upgrading, in my opinion a reflection of the carrier subsidy model for phones. But I'll keep things simple and just use the industry standard 2-year service contract as a proxy for the average lifetime of all these devices. Also as a result of this simplification, hand-me-down expansion of the user base would not show up in this methodology until those users upgrade their device with their "actual" first purchase on the record.
Certainly not all customers wait exactly 5 years to upgrade a PC (as this assumption implies). Some may do so sooner, some others may wait longer, and some simply don't upgrade at all (deaths, bankrupt businesses, and switchers come to mind). Further, the real average life of a PC is definitely not constant over time. Even though there are other ways to model replacement cycles for a customer already counted in the installed base, as well as ways to model churn (customer defection) and hand-me-downs separately, the simplified computation gives figures roughly in agreement with other estimates of computers in use at different points in time (see here and here) and is still useful for identifying saturation from the sales data alone.
We are so used to associating iPod with music that we've practically lost the actual general meaning of the word 'pod' as just a shell, a capsule, a protective enclosure, and most significantly a more agile transport vehicle of all the (mostly finished) digital stuff freed from the bulky and stationary computer, stuff we might want to keep with us at all times but couldn't.
An apt metaphor: just like dumb terminals were the trojan horse that enabled the PC revolution against monolithic mainframe computing, the iPod freed us from our desk-constrained computing needs, paving the way to mobile computing. It just happened to do it first to music, then simple contact information, photos, basic video, some very simple games, and only after gaining touch input, can now be seen as a full fledged computing device as part of the iOS platform and app ecosystem.
So as I mentioned, for this next chart I wanted to include the whole 1.5 billion user base for all traditional PCs compared to the yearly changes (y'≈∆y) which you already saw above in orange bars. Only now I've changed the color schemes a bit, and those orange bars are now shown in dark tones, with the total user base represented by lighter bars which get cropped in the last decade or so (left image, remember to click on it for the full version).
So now we finally notice the 3 distinct periods of expansion of the Apple installed base, with each expansion reaching a much greater number of customers. The first two completing their whole cycles of saturation, peak, and declines in growth (Apple ][ and early Mac), then a decade-long period of stagnation and declines for the Mac installed base, and then a resurgence over the last 7-8 years, bringing back exponential growth thanks to the adoption of Mac OS X, as can be seen by the rate of change continuing to climb straight up with no inflection to be seen so far.
I'll close this already too long second part of this series with a close up view of the last few years of Mac unit sales (now on a quarterly basis), broken down by those units that represent new additions to the installed base (green) and those that would be replacements (orange), overlaid with the y/y growth rate line charts for each, and the ratio of new-to-Mac over the total. I'll leave the interpretation to the reader.