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