Not true — Spark Harvest says otherwise. Granted, it was from a planeswalker-heavy set that really needed removal for them at common, but I don't think the ability to destroy planeswalkers is necessarily rare.

Not every deck plays planeswalkers, so adding some modularity through Cycling enables a player to be prepared for them while also not wasting a card slot. There's a two mana Plague Wind for planeswalkers in Standard with upside in The Elderspell that doesn't see much play beyond sideboards because it's a dead card against so many decks.

The mana cost might be a tad aggressive, but I don't think it should cost more than . Planeswalkers are becoming increasingly stronger, and as such, the answers to them need to become stronger as well.

Flatten is close