Based on product maturity curves readily available from business schools. All product sales go through phases. Early adoption, rapid growth, leveling off during maturity as unit sales become based on replacement rather than new adoption. Adding abilities, and trade off with completion will create spikes, but by and large overall phases will be seen.
Nothing new or startling in what I have stated. By opening new markets, China, India for instance Apple has attempted to continue the phenomenal sales growth it experienced in United States. The disposable income and fierce local competition from business as well as government protectionism have curtailed these efforts to some extent. Again this has been well documented, Feel free to hunt down the news articles.
As to what functions the hand held computer devices perform, I should think it rather self evident to any user. Beyond that not sure what "actual data" you refer to. Predictions are just that, data from future is not readily available, as crystal balls and time machines don't exist. So by the very nature of the topics, educated conjecture based on observations and past experiences are all Anyone can go on.
i'd be happy to entertain and discuss any alternative conjecture. Our hand held computing devices could be the one exception to past product sales. Perhaps people will continue to buy this product totally beyond need, replacing it continuously with ever increasing ferocity. Spurring growth to ever newer heights each year. Or sales could decline to a point the viability of Apple will become in question. But I strongly doubt either of these extremes will come to pass.