As often expressed - saturation of the market, which also gives rise to that special thing Apple try to cling to - "coolness". Like music, a general rule is : "If your parents (and grandparents) like it, it ain't cool!"
Wall to wall iPhones being used by grannies and grandads doesn't make the "kids" want them.
Android at least has variety in the eco-system - the opportunity to be more individualistic than a "me too! I got one, so I must be cool too".
I also fear that Apple, under the yoke of money-men and share-holders will sell their soul and head DOWN-MARKET. Jobs will be spinning, but wait for cheap iPhones and cheap iPads. There's nowhere else to go.
If EVERYONE can have a Porsche, it might as well be a Fiesta.
It's like when microwaves became widely available....when everyone has one and gets over the fact they cook things quickly......what more can you do?