I'd say it's "never within the current scope of the project".

I remember when Wikipedia had a two-digit article count. The plan was 100K articles. We all thought "yeah, right", but went to work anyway. en.wp is ~4.7M articles at the moment, and many topics we would never have imagined are covered.

So right now, the scope of Wikidata is roughly "what could be on Wikipedia, plus glue". A musician would not be notable if all he did was send off a single tape to a record company; neither would a self-published e-book with a readership of five.

But in ten years time, maybe we get auto-generated and seeded items from every publisher or national library for every new book. Maybe we've gone from 14M items to 700M (the same 50x factor as en.wp), and then these "never" groups will be "on their way", and it's "never" for astronomical objects and molecules (because for those, 700M won't even scratch the surface).