Prismatic Ending can exile cheap nonland permanents (mana value 2 or less, since we can only spend white and blue on it) like Grim Monolith, Giant's Boulder, or Disruptor Flute before they power out their top end, something our Swords to Plowshares cannot touch. Force of Negation lets us counter their Karn, the Great Creator, The One Ring, or Trinisphere for free on their turn, which is crucial against a deck that wants to resolve game-ending prison pieces uncontested. Stifle is weak here because mana abilities can't be targeted and Karn's ability is static, which leaves only a few triggered abilities like Urza's Saga's chapter triggers worth hitting, and Daze loses value since this is a grindy matchup where tempo matters less than raw interaction. Keep Wasteland in to attack their Urza's Saga, Urza's Tower, and other key nonbasics.