So, for clarification, I don’t think it’s either-or, it’s both and that’s where many user interface paradigms have room for improvement, being “too binary” i.e. are easy to use only if either everything integrates or everything is sandboxed.

Even if all of the customers opt-out of tracking, I don’t think ad rates would go down to zero. Google and Facebook should still stay profitable enough to provide these services. Besides, they can always restrict some features to customers who agree to tracking, as an inducement. (For example, I believe Google Maps app on iPhone asks for an opt-in so that you can use the addresses you’ve saved in your Gmail address book and it refuses to use the address book that exists on the phone, forcing the users to pick between opt-in and copy/paste addresses from Contacts app.) If they can come up with an even superficial technical reason why the features would require them to do some tracking, they can avoid customer backlash, too.

That, in turn, has interesting consequences to user interface logic evolution. Hence, somewhat counter-intuitively, people might not always want “unified everything” but actually to keep many things in their own sandboxes.