> serving that snippet from one's own domain is still pretty trivial, and indeed what most of the big ad brokers are moving toward

But in this case it's not as trivial to link usage across two domains. Sure, each domain can serve the snippet from its own domain, but the analytics snippet can't cross reference data from other sites which use the same script (unless of course the domain builds some data sharing mechanism - not trivial for non tech companies).

In which case why even serve the snippet form your domain when you can use the container mechanism.