Competitive market and downfall are not always synonymous...and i don't think "downfall" is a properly used term in this scenario. Apple isn't having a downfall, they are simply being rivaled now in a market they solely lead for many years. Plateau might be a better word...because i can certainly agree that Apple's strong hold on the market is plateauing, but i don't think this is a surprise to the company or the share holders...if there's one thing you see in the consumer electronics market, it doesn't take long for other rivals to mimic and even "reinvent" (for lack of better term) a product you essentially brought to popular view.
Apple isn't failing, they just arnt WAY out in the lead anymore, and that's primarily because they're not alone anymore on all fronts (phone, mp3, tablet, PC). Soon though, they'll venture into another realm, and they'll regain control in a broad sense...and even with the very competitive market, they still find themselves (most times) as front runners (though companies more easily nip at their heels these days).
Downfall is such an inaccurate terminology...Palm was a downfall, Blackberry LOOKS like it's still heading in that direction...there are others, and i think you get the point.