Apple's decline starts with its acquisition of Beats Electronics - popularly known as Beats by Dr. Dre - the headphones and music streaming business.
All-powerful brands have a life cycle (remember Pepsi?) and even the most hyperbolically successful ones fade eventually and sometimes fast (remember Nokia?).
Today's announcement of Apple's purchase of the Beats business and brand marks the beginning of the decline for arguably the world's most successful and valuable brand.
In buying Beats Apple is signalling that it has lost touch with the youth market - at least in terms of its music credentials - an area that once helped define Apple's coolness vs. the competition. The iPod defined the Apple brand for millions and then billions beyond the design/tech community who used and loved its computers. 'A thousand tunes in your pocket' was the proposition that Steve Jobs developed to transform the way music was consumed and even the way the music industry worked.
Staying in touch with the ever-evolving youth market is notoriously difficult. Brands like Coca Cola seem to manage it but they are the exception rather than the rule. Most brands that start off cool and hip grow up with their audience. Imperceptibly at first but eventually enough time and change happens that a few grey hairs start to show. Then the brand faces a choice - take the risk of reinventing itself and potentially alienate its core of loyal followers - or buy in some hip credentials that can be directed towards the seemingly fickle youth market at low risk to the core brand.
The latter may be lower risk today but in Apple's case it's a highly symbolic admission that it's either unable or unwilling to try and retain its power among the younger end of its audience - and once that corner had been turned there's no turning back.