My thinking is summarized as :
There is old data already stored with the Raw ambiguity. In lieu of having all old data be re-written, the solution is to keep that ambiguity and add new explicit types.
Any old code that comes in touch with newly serialized data will break, and will have to update their libraries.
However, the old serialized data will not need to change or have its interpretation change, regardless of assumption made on how to handle Raw previously.
Beyond that, we add new native type for Timestamp and PrivateExtensions.