What is the convincing advantage to centralize this in
rather than letting the wikipedia communities handle this independently?
The objective is to create an extension that manages all structured
data associated with any page, in a timely manner. There is already the
SMW extension that does specifically this, but it has encountered
puzzling technical resistance in the broad wiki administrator community
I know from experience. From a social view, their resistance is
explained by their sense of what their (normal) wiki users will
tolerate, and it likely doesn't include managing structured data.
more to the point is that NO extension need be installed for ANY wiki to
directly benefit from the sheer existence of Wikidata - via simple
transclusion of its resources. That cannot be said for the current
proposal by which it indeed requires wiki administrator concurrence.