How would the marines find the truth
Better yet, why would they care?
If you think Luffy will tell everyone, why should they believe him
Because Robin’s joureny isn’t about finding out this centuries old secret and just... not telling anyone about it. What kind of payoff is that? The Strawhats aren’t going to just let the World Gov continue to exist without telling anyone what really happened.
And “good” Marines like Fujitora will obviously care, since they aren’t overly impressed with the current World Gov anyway. Once they find out what was so bad that the World Gov are willing to commit genocide hundreds of years later to cover up is, they will care. Doflamingo only knows a
part of what they are hiding, and that was something he said would shake the world to its core and allowed him to blackmail the entire government. The full history is going to be even worse than that.
Imu, the Void Century, the real meaning of D, these are all things that the World Gov has tried desperately to keep a secret. That means the world has to find out the truth, otherwise the Strawhats are just perpetuating the lie. Everyone Robin loved died trying to find the truth, it’s obvious she’s going to tell it to the world in return. And when she does, people like Fujitora, Coby, Smoker all have to make the choice of what true justice is.