Well, look at it. Luffy may be propelled by Roger's legend, but now he has taken part in the Whitebeard war. Even though he doesn't have the raw power himself, he was able to prove himself a great man worthy of fighting by Whitebeard's side and was the key to Ace's freedom. Not only that, but what's going to happen here will cause an entire paradigm shift in how things are going to happen from now on. It's enough of a change to say that times have significantly turned from how things were previously.
If it were not for Whitebeard's actions, things would still be the same Yonkou gridlock and Marines running here and there. With the end of this war, Whitebeard, the strongest man in the war and the last relic from the past of great pirates, will be gone leaving a void in the Yonkou ruling. I suspect that this will trigger not only a race to conquer his territory, but cause a mess with the forces of the Marines. They will have lost Marineford and stretched their resources thin whilst the Supernovas of this era rise to meet their dreams and Yonkou scramble to control new territory, or a new one arises to take Whitebeard's spot.
A lot can, and will happen. So that, at least from what I gather, is why this will be known as Whitebeard's era. Yes, Roger's tone is carried into it, but Whitebeard's actions will have played a MAJOR game changer here.