Affinity relies entirely on artifacts (Urza's Saga, Mox Opal, Engineered Explosives, Kappa Cannoneer) so Meltdown is a devastating sweeper that can wipe their whole board and mana base at once. Tormod's Crypt doubles as free artifact hate against Urza's Saga triggers and Welding Jar shenanigans while also hitting their graveyard plan. Spell Pierce protects your clock or stops a key Preordain/Engineered Explosives. Boomerang Basics loses value since they have few key nonland permanents worth bouncing repeatedly, and the extra Lava Dart/Mutagenic Growth are less necessary once Meltdown is online as a sweeper.