This would be already a giant leap forward: Usually, there is not a lot of trust between actors, and there doesn't need to be: everyone gets their own tracking

This would be already a giant leap forward: Usually, there is not a lot of trust between actors, and there doesn't need to be: everyone gets their own tracking pixel and is happy, who cares if user data is leaked along the way. The setup you describe requires more trust beetween parties, and, as a side-effect, reduces the overall number of potential contractual partners which get access to user data. It also reduces the number of websites with the knowledge to implement such a setup.