Thanks Phil. I think I can try to put my finger on the ambiguity underpinning the difference in your interpretation of this principle and Matthew’s. With the emphasis on try! …
Christopher Allen’s principle is “Control. Users must control their identities.” I refer to the principle here as “Control. Users must control their identity”, singular. This is accurate (less ambiguous) because, as Allen’s accompanying explanation confirms, his use of the plural corresponds to the multiple users rather than to each user having multiple identities. Each user has one identity, and “it” is used to refer to that one identity.
If Matthew reads this similarly, then the one singular identity (referent) must, by definition, endure, for how else might they be referred to. It is not dynamic but static. It is an “it” among others, which together constitute a social system. Others refer to my “it” and I refer to theirs. They are then categorically a system-wide static referent.
If you interpret this principle differently to the point where you assert that it does not make either assumption, then perhaps you are reading “identity” as a mass noun. This is not an easy interpretation if only because Allen presents a binary choice (“They must be able to choose celebrity or privacy as they prefer”) that would not be required if one could — if this schema ever reflected reality — choose differently for one’s different identities.
I will pick up your question of control in your following comment.