Mass Polymorph

The whole risk of the spell lives in the word "until." Polymorph effects trade a creature you control for whatever the top of your library happens to surrender, and this is the plural version: exile your whole board, then dig until the deck coughs up the same number of creature cards. The exile happens first and unconditionally, which is the line that punishes a sloppy build. If your library is short on creatures, you keep flipping; if it has none left, you have just sacrificed your battlefield for nothing. The decks that want it answer the variance by controlling the denominator: run a tiny count of fat creatures, fill the rest of the list with tokens to feed the exile clause, and turn three 1/1s into three game-enders that dodge a counterspell because they never went on the stack as creature spells. That is the structural appeal. The bodies arrive from the library, not from your hand, so the answers that punish creature spells (countermagic, taxes, the whole class of "whenever a creature is cast" hate) miss entirely. What you pay for that is a six-mana sorcery that does nothing without a board and a deck deliberately stocked to convert, a narrow enough demand that the card has always lived in the build-around tier rather than the maindeck staple tier. It is the cheat-into-play archetype distilled to a single sorcery, with the upside and the self-immolation both scaled by how many creatures you commit before you cast it.