@King:
Our world is not like OP's world.
Piracy will still be rampant because everybody lives in isolated islands. Ships will keep being assaulted the same way our cars still are, since there's no other reliable method of travel available.
@Dylcor:
Will piracy simply die? In a world built specifically for it? A world made for sea traversing adventures? Even with the massive physical change every fan assumes the world will go through, I doubt piracy will die. Not when time and time again we see characters called to sea by the pirate flag, abandoning their families and duties for whatever the world has to offer them.
I just want to say that the world being built specifically for pirates is not a permanent state. The world is what we once known as the age of discovery and sailing is the real means of transport and because of this the difficulty in countries/authorities helping each other allows for pillaging and injustice.
The lack of a mode of flight and aircraft in the story allows for this (just like our history). So basically, what I am trying to say is with Oda's focus on technological advancements and Vegapunk playing such a heavy role, I won't discount airships and planes in the end.
However, if Oda wants to keep it at a permanent state of no-change, whether the writing allows that in a believable way in the future will be another issue.
Like for now, most of the fanbase rarely asks "hey so why aren't there planes" because the OP world doesn't have skyscrapers, bullet trains, industrial age revolution, etc. But we do know it will happen, like in every good fictional world that is based on the real world, a sense of progression for the civilization is a must no matter where the universe intends to go after it ends.
I'm lazy but off my head, the introduction of Kong above Sengoku means the World Government, while heavily invested in navy, does have a powerful military.
It's easy to forget that besides the WG, the world of OP is largely "governed" by the 170 countries which means that piracy is not as defined as we would believe.
In terms of raw power, yes, the pirates and marines can create chaos and destruction but in terms of civilization and societies, the 170 countries and Reverie happening means that this is not just a Marines vs Pirates cat and mouse chase.
I think the problem here is that we were shown how easily Crocodile, Blackbeard and Flamingo overwhelmed an entire country military and resources which weaken the countries governing stand in our minds but this is not the norm and they are the exceptions (3 shichibukai pirates).
Which is to say most countries don't easily fall to pirates.
Piracy is a means to wealth/fame/honour/power, just like how in today's world starting your own business is a means to achieve that. You can be the kindest entrepreneur around but you know that if you want to make it into the big leagues, you'll have to amass certain things but most people that successfully climb the corporate ladder at a fast pace is not kind (like Luffy is trying to do in the OP world). But if they are, then perhaps they are insanely talented (Luffy).
That is also why I believe that what Luffy and Roger said about what they want to do is probably to connect the world in some form or another, so that people from luffy's hometown village can easily experience the culture of skypiea or water7.
So you know, the weaker people who are not monsters like them don't have to risk dying in a storm just to experience the world.
The reason for that is because this belief of global human connection opposes the Celestial Dragons and corrupted side of WG, which indulge in misinformation and caste-ing people.
This ties in with that long long ago theory of the redline breaking up(or something like that) and all 4 seas to come together.
But that's another story.
Inherited will, dreams, all that kinda jazz is important to the story and the world, and it’s part of what makes one piece feel like one piece. I don’t think the series will end with luffy halting all piracy because he achieved it all. In this specific world there’s always gonna be someone who does more, and I think that’s how Oda is gonna leave us feeling. Not that piracy is gone, but that it’s waiting for the next king.
The next war will change the one piece world. Even if piracy were not to end (which I agree will not end completely), some big changes will have to happen after Luffy eventually "unite" all the good guys.
Most likely the evil in both WG and the pirates will be greatly reduced and peace will come.
If the next war or ending sparks another "WhiteBeard moment" then there's no point to this build up to Luffy as a messiah-like character or the "one" because it will lead to chaos.
Inherited will and dreams is nice but Roger also equally created chaos and increase overall pillage and rape in the world.
Luffy has to significantly reduce that or end it or there is no point to the narrative.
You are right that there will be another "king" but not in the same sense as what Roger and WB did, that's my two cents.