They already do "sell", in a very similar way as Google and Facebook does - look at their app store ads for example. Only real difference between Google/FB and Apple is that they process data on device, not on their servers. Sure, it's a huge technical difference, but not that much in terms of business and end users.
Google and Facebook don't sell your data directly, as it's bad business - you want to sell access to customers as a subscription to get big bucks. They have all the initiative to protect data they gather on their servers. Same as Apple has to do that on devices.
There're details to it, but just moving where you process user data isn't that much of a difference to end users - you're still a corporation selling other people ability to show me ads for cribs, when you discover that I'm expecting a baby.