They key to Apple's success so far is that it has kept moving into new territory. It's not still making just Macs, or just iPods, and it didn't even stop with the iPhone. And it wasn't afraid to be all right with those other products outselling what was perceived as its "core" business.
This is different from other companies. Palm stayed Palm, and didn't branch out much beyond PDAs and smartphones, and took forever to even evolve its operating system. Microsoft tried branching out after it saw that Apple was successful, but by and large, those were just viewed as copycat side projects. And Google innovates slightly more, but has software engineers driving the design, not people who are attuned to how users will use their products. And so everything is beta for the longest time, and the whole "it just works" philosophy that people come to expect is thrown right out the window.
And Facebook is still Facebook. It hasn't changed all that much, except for a few user interface tweaks and policy changes that always seem to annoy its users.
As long as Apple keeps experimenting and branching out, then it should be fine for quite some time. And in any case, there's no need to go planning your exit from iOS and OS X anytime soon, if that's what you use to get things done.
In any case, let Apple worry about where its future is headed. If they lose their position at the top of the innovation food chain, it'll be because someone or something else has bested them. And THEN you can worry about e whether it's time to move to a new platform.
