The number one concern about Apple is how future versions of the iPhone will sell. Will the company be able to add enough features and functionality that current owners will want to upgrade? Will Android users switch? Will feature phone users buy Apple vs. Android? A longer term concern is will another company introduce something that changes the landscape like Apple did to Blackberry that instead of a slow erosion that sales fall quickly. Parks Associates’ recent study of consumer electronics buyers I believe supports the case that the iPhone and therefore Apple will remain successful for at least the foreseeable future. (Note that I own Apple shares).

From the article "Why Apple's iPhone Is Too Big To Fail" by Chuck Jones.