To be clear, this proposal violates the "Source Stability" and "Control of the Call Site" principles proposed by the core team at the beginning of this thread. I think you are arguing that eventual consistency of the model is more important than those principles. I consider that a hard sell given the importance of source stability for the overall Swift ecosystem. Source stability is a hard constraint to stomach, because it means we must accept that some mistakes we made in the past can no longer be fixed, but Swift developers have told "us" (as developers of the Swift language, core team members, etc.) repeatedly that it is important that we not break their code from one release to the next.
The core team has stated that they do not want to break source compatibility here, which means that existing code needs to continue to compile. API authors are free to decide themselves whether they want to adopt a feature that will break their clients; that's outside of the scope of the language's source compatibility.
Doug