In my opinion "how many items will X add" is a false problem. If p.a. we
moved file categories to subpages as we do on templates, we'd have 20
millions new Commons pages: but the question would be, are they as
accessible as they were before? Similarly, the only danger is when
items' statements are not transcluded outside Wikidata.
When stuff is in use on projects, as for authority codes; and when it
can be edited in-place, as we all do for sitelinks and ru.wiki does for
much more: then there is nothing I worry about.
Nemo