Too simplistic of a view.
First off, no, Orochi is not the complete opposite of Luffy, he is the complete opposite of the Kouzuki within the context of Wa No and samurai and Oden in particular, something Hiyori underlined in her revenge speech recently.
The conflict between Luffy and his values on one hand and Orochi´s on the other has not once been incorporated or underlined.
Him being a sniveling, cowardly weak rodent has no effect in that regard.
And the Kouzuki and Luffy/Joy Boy are not synonymous with each other.
To equate that to the World Government who has been the only constant antagonist and connected to each arc one way or the other (either through history, the prevalent society structure the WG has created, forces of the WG being directly or indirectly involved) is groundless, and who have been, time and time again, have been the driving factor behind Luffy´s opponents, and literally oppose everything that Luffy stands for, cares about, represents or whatever else you want to call it.
Luffy is the personification of freedom, literally underlined even further in the recent chapters with the freedom to fight however you want (regardless of whether it´s a development to compliment), the force that has been oppressing people for centuries is the WG who immediately label anyone who even remotely wants to break out of said societal structure and wants to walk their own path, a criminal.
Since you talked about Kaidou, another factor is also crucial. Compare fights of Luffy vs Kaidou or Katakuri for example, characters that in their own right and way, in some ways conflicting with Luffy´s, tried to achieve their goals and dreams and how Oda developed those fights and Luffy´s demeanor vs antagonists whose primary goal is to take away people´s freedom one way or the other.
It´s literally a competition of people on a particular spectrum of good/bad that still are looking for the same thing, achieving their dream and their form of freedom. And Luffy´s demeanor in those fights mirrors that aspect, something people falsely criticized for Luffy lacking the necessary seriousness since he smiled. But the participants enjoy competing with each other for the same thing.
Now take fights like Luffy vs DD, Luffy vs Croc, Luffy vs Lucci and so forth and look at the change of Luffy´s demeanor, how he approaches the fights, and what he actually articulates.
One meaningful example:
And it´s no coincidence that Luffy says this to someone who both literally and in his values, a CD, someone who has the same disdain for people and their freedom and their dreams as the World Government has.
And i am sorry, at least so far, being a dark version of the same thing just does not have the same relevance thematically.
At best it underlines that the dark version of the protagonist might be the best contender for the thing the like minded people aim for, but all these people in their pursuit are antagonized by the WG as well as the constant that opposes those values.
Now who knows how Oda will develop this further, maybe he might turn Blackbeard toomeone who is literally trying to put the world into darkness or whatever, maybe Luffy beats him for the PK title, but BB gains another power or whatever but right now, it´s by no means what has been shown.
And regarding the Roger+Garp vs Rocks thing, for one at that point Roger, as far as i remember, had no clue about anything regarding the true history. I might be mistaken but i remember that their last voyage took 3 years in order to reach Raftel, and that before they reached the last island before Raftel, and then there learned about Ponegylphs and whatnot, meaning the fight had no background of history and so forth.
On the other hand, i won´t deny the fact that if Roger regarded Rocks as the bigger opponent over the WG, it would legitimize Blackbeard further.
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