You can measure conversions without relying on personally identifiable information. Adding e.g. a non-personal tag to your URL will enable you to track the source of a conversion. If this happens in a browser you can even store that (non-personal) information in the localStorage or a cookie, so even if the user doesn't convert immediately and only comes back after e.g. a week you will be able to tie the conversion to the specific ad. Where in that process do you need personally identifiable information?

Also, Facebook doesn't only use these identifiers for conversion tracking, it uses them to create a persistent profile of a person that often ends up containing a ton of sensitive information. Believe it or not, a lot can be inferred about you from your app downloads and website visits. As of now FB was mostly able to perform this kind of tracking without the user really knowing, the only thing Apple does here is drag this practice into the open and give users a real choice.

In my opinion a privacy-first OS should inhibit any kind of cross-app / cross-device personal tracking, I predict this will happen in the next 5-10 years, for now it seems to be too extreme still.