Everything has a peak.
Apple may make obscene profit margin for the next ten years, but at the end of the day they have very few products and are vulnerable to fashion. There hasn't been a compelling reason to buy an Apple product for maybe 15 years. Video editing, Photo editing, Desktop publishing, Tablet Internet consumption, phone usage, Pocket Music player etc can all be done just as productively more cheaply on many other products, with many manufacturers offering more variation of models in price, features and style.
Apple are arrogant and offer little choice. They have been more successful than their product warrants on ipod, iPhone and iPad, but eventually those will be niche products just like iMac and mac Air, unless there is a change in Apple Corporate philosophy. A change in Apple marketing and pricing philosophy could backfire and cut profits even if market share increased, and could even reduce market share.
There is no doubt a large part of the success of Apple has been to hide complexity and make the products seem simple to use with a consistent GUI that doesn't change pointlessly with each release (Hello MS and Ubuntu!).
Perhaps eventually Apple products will become higher quality and more expensive and they will settle down to a 10% peak market share with high margin. This is probably a better strategy that trying to have a majority share and offer cheaper products where you can easily fall off a cliff (Chrysler, MS Win CE PDA, MS Win phone, Nokia, Sun, Wang, DEC).