In the world of technology, it's not if the mighty will fall but when. There are to be sure, a few exceptions to this rule: IBM, Sony, Oracle and a couple handfuls of others to name a few. But it's a small list, and not one is growing by exponential measures.
If you don't think declines possible, look at Microsoft's history.
There was a time when the government and everybody else wanted to break up the big evil Redmond. Bill Gates and company were brilliant. They came up with this idea of having tollbooths on the places where we did most digital commerce. they owned the operating systems on the computers were used and we paid them for that. They owned office productivity software where did all our work and we paid them for that. The almost did it with web browser by wedding Internet Explorer with Windows operating system, but then came along Firefox and Google.
So you say that IBM is well, but there was a time when Big Blue was scheduled to be broken up into little parts just like the original AT&T. Then, a funny thing happened on the way to the disassembly party, IBM got a new CEO who thought better of the idea. Yeah, they sell off some parts, like the PC division. But for the most part company still in tact, but with a higher reliance on high-margin products like services.
My point here is this, if you can't trust Big Blue or big evil to stay at the top of the who can you trust?
I'm not saying that Apple will inevitably go downhill. I hope not. We have three iPhones in my family and I love the iPad. And apple has always been a company that gave consumers what they wanted not what they asked for. But there are no sure bets in technology, so invest in Apple, but make sure you buy little Exxon-Mobile stock as well.