The greatest problem with this supposed reason is very simple.
At no point has Luffy ever established his opinion on what exactly he perceives “freedom” to be.
If one were to generalize the concept of “freedom” then you would find two basic types.
I) Freedom from. This is the desire to leave behind that which one deems obstructive to one’s happiness/quality of life in some way.
II) Freedom to. This is the desire to do what one seeks to achieve in life which in turn will contribute to one’s happiness/quality of life.
These may seem similar but they are different.
For example, through being a pirate, Luffy is no longer bound by any law of any nation thus he has
freedom to do anything he pleases.
However, in becoming a pirate, Luffy is now unwelcome in most civilized territories and is consistently hounded by the Navy and bounty hunters. As captain he is also responsible for the burden of protecting his crew and giving the final order on what course of action to take. Benn Beckman pointed this out to Luffy (albeit somewhat jokingly since Luffy was a child at the time).
In short, Luffy has denied himself
freedom from both increased responsibility and the entire judicial/law enforcement system.
The problem with all this is that all Luffy sought to do is have nakama and adventure which he could have done without being a pirate.
So
why did Luffy desire to become a pirate?
When he lived with the mountain bandits he was completely free from any and all typical societal responsibilities. The World Government did not even know of his existence. So what was there for him to seek
freedom from?
All Luffy has ever stated to desire (beside the tenuous goal of becoming the Pirate King) is a loyal crew and adventure. The thing is, there is no law or societal responsibility preventing Luffy from doing this legally.
For example, Montblanc Noland was a famous adventurer, funded by governments to find new land with the assistance of a loyal crew. In fact, even becoming a Marine would have allowed for both adventure and a loyal crew. So what
freedom to was Luffy seeking?
Again,
why did he desire to become a pirate?
Also, why does he perceive the Pirate King to be the “man with the most freedom” when seeking such a title means upholding a duty to crush any challengers to the title (of which there are many) as well as being the most wanted man in the world thus being hunted down with extreme prejudice by all law enforcement organizations?