The real threats for Apple, imho, are successful web companies expanding towards hardware. Apple does great HW and poor online services; someone doing great online services and great HW could hit them badly. Google got close already; Microsoft will try again once they complete their retooling; Amazon are unfocused but could fish a winner with some original thinking; Facebook/Dropbox/others might also challenge at some point. Luckily for Apple, hardware is very difficult to get right and get profitable.