Eh, they own the high end. Netbooks have been around for years, still no one can undercut apple even with ultrabooks.
I just don't see any long-term growth opportunities for Apple's model. Their computer and notebook sales aren't a dominant part of their success, and those products aren't even really 'high-end' in the traditional sense, although the aesthetics and packaging are quite good.
The real bread and butter is the huge profitability of iPad and iPhone, both of which will continue to face increasingly brutal competition. The imminent $199 7" Kindle Fire is a hint. Imagine what a slightly more capable 10" model at $299 would do. The writing is already on the wall for phones, and the big cell companies who back-end the pricing make more money by shoving a $300 Droid into a $49 or 'free' contract agreement than trying to do the same thing with a $500+ iPhone product. Apple is bleeding marketshare in the phone biz.
I don't think it's some super doomsday scenario for Apple, but I think within 12 months the picture will be much clearer, and certainly within 5 years Apple will probably be a fraction of its current size unless major innovations are made.
Cheaper technology pricing for components (flash memory, processors, screens, batteries), along with increasing average quality, and a very cutthroat competitive marketplace all spells a repeat in the smartphone and tablet areas just as occurred in the PC market, TV market, etc. Probably within 3 years at most we'll see legitimate $99 droid or other non-apple phones that are more powerful than one would really even need in a phone, and probably in the $200 range for tablets of that nature (quad core cpu, 64gb+ ssd, 10" hd screen, cpu-integrated gpu acceleration, etc).