4c Artifacts relies heavily on artifact enter-the-battlefield triggers, such as Simulacrum Synthesizer's scry and Construct creation and the exile or damage effects on Perilous Snare, White Auracite and Spring-Loaded Sawblades. Karn, Argent Defender shuts all of those off, including for permanents they put onto the battlefield with United Battlefront. Terminal Criticism only destroys blue or red creatures, and their mainboard has none (their tokens are colorless, and Krang is the only blue creature they might bring in after sideboarding), so it stays in the sideboard. Duress is the better disruption here, since nearly all of their key cards are noncreature, nonland spells and we can take the best one before it comes down. Shoot the Sheriff is less necessary because they have very few creatures to kill and shutting off their artifact synergies matters more, so it comes out. Dissection Practice's small life swing is replaced because we need harder disruption against their grindy artifact value engine. The rest of our proactive curve and full manabase remain to close the game quickly.