I haven't seen TF:TM, but apparently they need the matrix of leadership to destroy Unicron. I did see the TFP finale. Do you think they could destroy Unicron by sending him into a sun? Or sending him into a black hole?
Nah, because he's too damn strong. Only the combined wisdom of the Primes, the descendants of Primus, contained within the Matrix of Leadership can destroy Unicron.
Historically, they've actually TRIED both of those things. Per the Ultimate Guide, after his defeat by the Thirteen, Unicron was blasted through a black hole, but it only wound up depositing him in another universe. And in Energon, they tried to trap his spark within a sun, but he was so powerful that it caused the sun to collapse into a black hole itself that started sucking up all of reality.