Devastating Onslaught

The doubled X in the cost is the shape of the whole payoff: because you pay X twice, every two mana buys one copy of the target, a linear return that scales cleanly with the mana you can dump into a single turn. Aim it at a creature and you rent a hasty swarm, an alpha strike that materializes and swings the same turn it resolves. Aim it at an artifact and the calculus changes: the tokens only last until the next end step, so the value is not the permanents but whatever they do on the way in, the enters-the-battlefield triggers, the tap-for-mana or tap-for-effect windows, the sacrifice fodder they leave behind before the mandatory cleanup collects them. That end-step sacrifice is the tax that pays for the copy count. You are borrowing an army, not raising one, and the card is honest about it: the tokens gain haste specifically so the loan has time to matter before it expires. As a sorcery it wants a board and a mana engine already assembled, which frames it less as a finisher you cast on curve and more as the release valve for a deck that has stockpiled resources and needs to convert them into damage or triggers in a single turn.