Well how else are they going to keep growing if they don't extract ever increasing rents? Realistically they've [1]plateau'd in market penetration for mobile phones, so their push into advertising, jacking up costs for established services is really MBA 101.
MBA 102 says you always maximize profits. You don't set a goal and then diversify when existing revenue streams plateau, you seek new revenue streams all the time.
And I don't think iPhone has plateaued anyway. Worldwide, iOS market share has gone from 20% to 28% in the past five years[1]. Apple is working hard to accelerate that growth. They may or may not succeed, but it would take an MBA 101 dropout to accept a 28% market share plateau.
[1] https://www.businessofapps.com/data/apple-statistics/