If you're playing Snap and Isochron Scepter, it's quite obvious you're playing control, so you'll have an abundance of counter spells (FoW and the like) already so I don't think the "return target spell" will be very usefull
UR Ultra Rare Gandora-X the Dragon of Demolition X DARK Level 8 [ DragonEffect ] ATK 0 DEF 0 When this card is Normal or Special Summoned from the hand: You can destroy as many other monsters on the field as possible, and if you do, inflict damage to your opponent equal to the highest original ATK on the field among those destroyed monsters (your choice, if tied)
While many cards will leave the format and slow down the format significantly, some of the game's best decks will still thrive as they will find new tech options to play to replace the cards they lost
Blue decks have to win the game eventually, and Delver of Secrets provides an easy-to-cast win condition for spell-heavy decks
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So running too much mill is extremely counterproductive, as it vastly shortens the deck's staying power in exchange for a bit more explosiveness