When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell
The Replenish deck could sit back, lose card advantage turn after turn with Attunement, and still come out and win at its leisure as Parallax Wave, Opalescence, Seal of Cleansing, and other enchantments all entered play at the same time
Its biggest tricks rely on making Illusion cards Vanish from the field, only to return to your hand, gain some insta-lore, or manipulate the board state
While they read like niche color hate, the absurd efficiency of a one-mana counterspell/one-mana removal split card elevates this to an S+ sideboard card for red decks, and a mirror-beater for any blue deck capable of fetching Volcanic Island which is all of them
Blood Baron is also completely absurd against any deck that pretty much cant touch him, such as Aristocrat decks or B/G Midrange