Thematically, it's really important that Luffy's the one to unleash the haki blast that saves Ace. In other points in the manga we see that Whitebeard does in fact have Conqueror's Haki. The majority of this chapter has been spent building him up as the ultimate badass, spouting one-liners like, "I am Whitebeard" and making statements of guiding his sons towards a brighter future, but at the end of the day he can't. He's too old, too sick, and too injured to accomplish his desires.
But Luffy, the shining light of the next generation, can. He's spent most of this arc getting thrown around like a rag doll, but when shit hits the fan he's the one who ends up saving the day. The torch is being passed, not from Whitebeard to Ace as the WG feared, but to Luffy, and it's why Whitebeard speculates that he might be the one destined to inherit Roger's Will.














