Black Lotus even back in 1994 was incredibly expensive
Now, I know what you might be saying
Milling started on an artifact, and as such much of the follow-up milling effects were also put onto artifact cards
It's like she knows she can do good art, but enjoys fucking with people by giving us complete crap to look at instead
Of course, the biggest upside of Zirda, the Dawnwaker is that its cost reduction applies to all permanents (see: the Basalt Monolith / Grim Monolith combos for Commander), and some artifact-based decks come very close to accidentally meeting Zirda, the Dawnwaker's companion restriction (for example, Modern Urza decks often only have Uro, Titan of Nature's Wrath as a permanent without an activated ability), so the unique wording on Zirda might open up new possibilities, compared to Biomancer's Familiar and Training Grounds, that will take time to explore