Tell that to Nokia, they too tought they did not need to innovate and could rely on the brand...I don't know if you need to worry about innovation if you have a brand name like Apple or iPhone to market.
Fact is, no matter how big you are and how popular your item / brand is, sitting back is not an option, else you fall down, and the higher you are the harder you fall (again think Nokia and ...well Blackberry).
I am not saying Apple does not innovate, because it does, but your statement is wrong there, they should worry if and when they start to stale.