It should be of major concern. Phone players only remain on top for so long. Nokia, Motorola, BlackBerry, there are many examples throughout history of the rise and fall of the “smartphone giant”
all it takes is one innovation, one design to really catch the consumer’s attention, and then it’s a slow 5-15 year fade into obscurity. Apple is so large it thinks this could never happen, but it very much can happen...
Obviously all companies are vulnerable. In this case, Apple has proven to be able to disrupt their own businesses in order to focus on the next thing.
Macs used to be the money maker for Apple. Then the iPod took off and they stopped focusing as much on the Mac. They knew the iPod would be a bigger business. They developed services like the iTunes Music Store for the iPod.
Then they killed the iPod business in favor of the iPhone. They put all of the features of the iPod into the iPhone. 95% of iPod users no longer needed an iPod. Apple saw the future and skated to the puck.
So whatever the next big thing is, I don't think Apple would think twice to cannibalize the iPhone market in favor of the next big one.
The other companies you mentioned above didn't have a culture that included killing off businesses that were their dominant thing. Look at Motorola in particular (I'm quite knowledgable about MOT as I've been an unlucky shareholder since forever). They dominated cell phones in the analog days. When digital was coming around, their internal divisions fought for power and they made their bet on analog. When digital became the obvious choice, they were late to market by 2 years. They never recovered. Horrible leadership and horrible culture.
Blackberry also had a similar fall from grace. Their money maker was their email servers and business clients. When the iPhone and Androids came on the scene, they thought their moat would protect them. They didn't embrace the "full web" experience nor did they embrace the inevitability of apps. Now look at them. Business customers didn't want to carry two devices. They lobbied their IT departments to allow iOS and Android to hook into their email servers. That was it for BB.
Nokia's a little harder to figure out. They should have been able to compete. They completely dominated mobile phones between mid-90s until iPhone/Android came into their own. They were a little slow to embrace the mobileOS model that we know today, but they were in the game with Symbian. Not sure why Symbian wasn't further developed, but when Nokia realized that they were in trouble, they took the wrong gamble with Windows. They really could have leveraged their top notch design and dominant brand to market a skinned Android, but I guess they didn't want to be just another Android vendor.
Anyway, Apple has a history of being able to skate to the puck. They should be fine for the next 20 years.