Apples income is the product of
1. Profit per unit
2. Total size of market
3. Percentage of market they have
We know that 1 is very high due to the gullibility tax,
Number 2 is still increasing merrily but only until everyone has a phone. Then it's just replacement only.
Item 3, however, is dropping.
If any of these items drop too far, Apples profits will suffer. If number 3 keeps dropping, it could in theory get to zero and that means Apple gets no money. This is not going to happen. There will always be uninformed people with money to burn but it could go down so far that their only option is to change item 1.
If they drop the profit per unit to try and increase sales, they may loose more customers as their products become less exclusive and "special". If they increase their markup, how long will their user base remain loyal?