Doomsday wins through a single explosive turn protected by Force of Will and Daze, so Defense Grid raising the cost of those free counters by {3} outside their turn gives you a much better shot at resolving your own combo pieces or holding up interaction. Consign to Memory can counter their colorless spells like Lion's Eye Diamond activations is irrelevant since that's an ability not a spell, but it does stop colorless spells and triggered abilities, making it a cheap way to disrupt pieces of their combo sequence at instant speed. Omniscience is too slow against a deck that can kill out of nowhere once Doomsday resolves, so it's cut along with Acererak the Archlich, which does nothing to interrupt their combo.