Unicron is trying to destroy the multiverse, but he's still vital, because if he's destroyed, than another Multiverse consuming black hole will emerge.
PS: Infinite power supremely surpasses Galactus and LT, and you still can't counter the evidence that Primus has Infinite power.
(here is some evidence)
1. Every time he puts a tiny fragment of his power in an object ( ie. cyber keys, matrix etc. ) it never runs out. EXAMPLE: even after the cyber keys ( which contain tiny fragments of Primus's power ) were used to destroy the multiversal threatening black hole, they were still used to created a universe spanning space bridge.
2. Primus never seems to run out of energy. Even after billions of years, the most he got was just low. Then he went back to full again.
3. Unicron is capable of surviving in nothing for eternity. ( which was said as his goal, but I don't remember where. I'll try to find it ) which means that he never runs out of energy (infinite). Since Primus and Unicron are equal in planatery forms, then Primus kinda inherets that ability as well.
The Transformer's sparks are all tiny, tiny, portions of Primus's power / spark. And since the Transformers sparks last forever ( hence the fact that the TFs can't die of age ), that strongly suggest that Primus has infinite power. ( since tiny, tiny portions of his power last forever / are infinite )