Great point. Apple is lauded for their 5-6 year of iOS updates but it's not an altruistic move - the updates enable users to keep using their apps/services.

Also there's an upper-bound on price for an iPhone Apple is able to charge and if I can speculate, the "Pro" lineup does not outsell the "base" iPhone significantly. For one, there isn't enough product differentiation for an average consumer to pay the $300/40% price premium.

So in the world of 3-4 year iPhone cycles, keeping up the walled garden and monetizing those within is the surest path to revenue growth.