I think you missed the point. No application developer wants to use "sub" as the identifier; they want to use "email" or "phone" because these a) are actual ways to message a human and b) do not require a deep understanding of any technical spec to do the intuitively obvious thing.

I am not saying that my solution works today. I am saying that is a completely natural thing to want and the fact that it doesn't work or that we're even having this discussion is failure of the people who designed and implemented these specs.