Since Zirda, the Dawnwaker only cares about permanents, it could show up in a deck that is mostly (or exclusively) spells (people have mentioned this as a possibility in Vintage, where some decks incidentally meet Zirda, the Dawnwaker's restriction, although whether or not getting a random 3/3 in hand to start the game is worth a sideboard slot remains to be seen)
They can still cast spells, so if they have a way to remove Overwhelming Splendor with a card like Krosan Grip they still can, but if it's tied to a creature, your opponents can be stuck under the Curse for the rest of the game
Unfortunately, since the card's included in the Commander banlist, and isn't you're unlikely to experience the joy of trading a treasure for a Blightsteel Colossus - though you're allowed to run a single copy in Vintage, if you've got the cash
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