Isn't this just going to drive all the tracking serverside?
Hash up the tracking data in the app, send it back to the app's servers, send it to whoever you like? Same is happening with browsers and 3rd party cookies.
Sort of. I was thinking how tracking will respond now. And the answer is known: device fingerprinting, i.e. collecting all accessible device details to uniquely identify it. But since Apple fights fingerprinting too to certain extent, this sort of tracking will not be as accurate as the current identifier tracking. Also, it is significantly harder to implement. So, some monetary losses for the likes of Facebook are given.