@Lady:
Well you've sullied the reasoning here. The now extinct great kingdom, which we are to believe was both powerful and peace loving, would have naturally wanted to expand out of both instinct and morality.
I'm not sure how much history you know, but kingdoms don't expand through peaceful means unless they're marrying people. I don't know what moral grounds are covered in being an Empire because I can't think of any that have been acceptable since roughly 1960.
The growing Empire trope would cover an evil place. The peaceful idealistic kingdom doesn't do things like that unless it's being corrupted.
So yeah, a kingdom out to commit horribly disruptive environmental disaster in order to take more things over….this is a villain you are describing!
We can assume the then WG (coalition of opposing kingdom's) was not the most humanitarian of government's and may have well been lacking in technology to the extent where citizen's standards of living paled in comparison to the extinct kingdom.
You're framing this like the poor undeveloped kingdoms resisted the imperial kingdom's advances.
This makes them sound like heroes lol. Good for them!
Efficient expansion in this theory would have required the elimination of the two great obstacles to continental travel, the grand line and red line. Another Kingdom's expansion is a very threatening scenario for any other existing kingdom's hence opposition, cue war etc.
So yes again, to reiterate, the all conquering empire with huge technological powers over the rest is out to violently alter the world's entire ecosystem.
The Ancient Kingdom is a monster!
Real world logic on ecosystems aren't applicable to a story such as One Piece where we have islands made of dessert…
Except Oda has included exactly those sort of systems in One Piece, they even rode a sea current to Fishman Island.
And that ignores the part where I pointed out internal One Piece logic, of the Grand Line (where like 90% of the strangness is) means that the end of it…would destroy and otherwise horribly mangle many many of the islands there.
Also when the heck was there an island made of dessert?
Convenient sailing = Efficient travel = Globalisation = ???
One Piece is already an extremely globalized world.
It's not just about the sailing.
Ok, so we have a poorly understood reference from you about globalization (which has downsides btw).
And….what else do we have.