"no growth left in selling phones" is kind of the wrong way to look at it. Even if their number of units sold per year never grows, it's a very healthy revenue stream because they manage to convince existing customers to buy new hardware. Considering the cost per phone the revenue per customer is pretty high just from selling phones if they can get them to buy every 2-3 years.
Naturally, the 30% take from the app store is bigger, but that's a matter of overall volume, not growth. The app store is a cash cow and will remain so even if its numbers start trending down.
That 30% matters to them a lot.