Trafalgar Law, watching Luffy commit to crushing Doflamingo himself, spells out the strategic cost: the alliance’s original plan was to leave Doflamingo alive precisely so the severed SMILE supply would detonate the Doflamingo–Kaido partnership (“let them fight”). By taking Doflamingo down themselves, they instead inherit the Emperor’s wrath.
Earliest known caller
(TBD.)
Why this matters
A textbook chekhov-explicit future-debt install: rather than foreshadow obliquely, Oda has a character state the forward stakes outright in dialogue, converting a local arc victory (Doflamingo’s defeat) into the engine of the next saga’s conflict (Kaido). The “let them fight” plan being scrapped is the lever that aims a Yonko at the Straw Hats — the structural bridge from Dressrosa to the Yonko era.