Zoro/Oda is telling us that the force that together with the warlods was equal to 4 Yonkou is too much for a single crew, this already tells us that the Strawhats can't and won't solo the navy, Oda didn't create Cross Guild for shits and giggles, the revos have been in the story since the beginning.
One thing a know, after fighting a Yonkou with 3 DFs, a D that might be related to Xebec, Akainu would be a massive downgrade and there's no way he's stronger than BB.