him being the quintessential pirate to the average schmuck already sets his importance at a higher degree than Xebec's. But I reckon you don't count this to the story's importance, even though the main premise of the story stems from this. But let's ignore this for now.
You seem to be misunderstanding the concept of One Piece.
The treasure's originator is Gol D. Roger, it's not joyboy or any other chump.
There is no rediscovering of One Piece, the idea of One Piece didn't exist before Roger.
Luffy is carrying Roger's will. An individual can carry the wills of multiple people. I don't know why you would say "rather carrying joyboy's will".
As for attack on God Valley, we already know the sole reason they attacked it was because they were there to attain something which Newgate first wanted to confirm that it was even there before they got into a fight when they landed on God Valley. (More than likely they came for Kaido's DF and the other, since it gets hyped up by Ginny right before they arrive.)
They're attacking civilians... Outside of Newgate and Goloriosa rest aren't exactly all that nice... Kaido and Big Mamu are some of the biggest genocidal freaks we know,
Also the fact Roger didn't have any intention of fighting Rocks, and rather even told Rocks to wait.. but the latter chose to fight still.
In the very chapter, where Sengoku in front of Brannew and others hypes up how big and dangerous xebec was, how he opened assaulted the WG... the pirates that the World Government considered the biggest threats were still... Edward Newgate and Gol D. Roger.
No, it's not Roger's treasure. It is simply known as his. Thanks to Roger's last words, the One Piece gained fame and attracted the attention that hadn't been able to since the Void Century. But what awaits at Laugh Tale is what Joy Boy left behind. Roger named it and popularized it. Without Roger, there would never be any
race for the OP. But the OP itself would still be there--it's JB's safeguard against the world forgetting what truly happened during the Void Century (plus, most likely, a way to revert the results of IMU's dominion).
You are misunderstanding me in regard to in- and out-of-story importance.
Roger is an important figure. To the world & its inhabitants. To the MC. And to us. He is the epitome of what Oda believes good pirates should be.
In-story Rocks may no longer be remembered, and by EOS his name may forever be deleted. But Oda is telling us how he was important for what he tried to do. By his own charisma, he's put together a formidable group that's still influencing the OP world 40 years after his death. Rocks' unattained goal is still being echoed by BB and, in a more twisted way, by Dragon himself.
Every OP arc has been about an evil monarch having stolen the country from the rightful heirs, and Luffy righting the situation. IMU, Joy Boy, and the Void Century are this on a macro level. IMU is the evil king who must be dethroned. Roger and Rocks were both proto-Luffys trying to dethrone IMU in
completely opposite ways. Both failed. Rocks used raw strength and a straight-forward assault. Roger tried using the truth left behind by JB, but fate wasn't on his side. Luffy will succeed by combining these two approaches.
They are the checkered fate of the D. clans. Roger and Rocks. Adventure and destruction. A D. who minds his path, but by doing so still becomes a menace for the WG; and the D. who openly opposes the WG in the most violent of ways. The true spirit of the story is the romance, the adventure, the want to discover, explore, learn. Rocks runs count to this. Rocks is the "conquering" part of piracy, including by mass exterminating those needing to be conquered.
Again, it's the same result. Adventuring around the entire world, only to at the end find out the truth about the WG, the true history, and the demons now in charge of everything and deciding to right this wrong is the same as going to Mariejois from the get-go and exterminating those in power and their servants. The means differ--and of course there's the whole issue of morality at play. But
what Roger and Rocks wanted done is the same, just not the
hows.
The Rocks Pirates were 10 psychopaths who decided to take everything they wanted by force. Roger potentially invited
every psychopath in the world to sail to the very end and obtain a treasure able to give them control over the entire planet and fuck whomever stands in their way. Yes, Roger only said this in the hopes of inspiring the next little Rogers and Joy Boys, but that doesn't mean that the Blackbeards and Caribous didn't hear it and heed it as well. In the long run, it's Roger's words that will cause the greatest mayhem, and not Rocks' violation of the Geneva convention.
True, one can carry multiple wills at once, an entire people's will even. Luffy took to the sea after Roger's example and appears to share Roger's own, secret dream. But by following in the PK's steps, Luffy is fulfilling the will of the first pirate--Joy Boy--setting right the injustices carried against JB and his crew, and even going as far as unintentionally keeping JB's promise to the fishmen. This is all JB's path being followed by Roger first, then by Luffy, with Luffy alone being able to fully complete it, thus making him able to surpass the first PK.
Lastly, Roger may be the most important pirate (for now), but that doesn't mean Rocks couldn't have been more important (and dangerous) to the eyes of the WG than him. After all, currently in the OP universe the most dangerous man is
not a pirate.