> The part I'm not getting is why this thread is discussing application-specific requirements for structural constraints on data
> without reference to BIBFRAME profiles, which are _all about_ specifying structural constraints on data.
Because this thread, to my understanding, isn't about structural constraints at all, and never was. Structural constraints are an interesting topic, but as I understood Rob Sanderson's original critique, the question of "types vs. properties" isn't about constraint at all. It's about extension.