Knowledge vs. Power is the key for ruling an entire civilisation and was Joyboy's mistake..

Joyboy was a powerful man, but never Wanted to rule a civilisation as a King, as a Leader and therefore offered his soul to the Devil to Make the required sacrifice.. All he was speaking about and Looking after was freedom, connection to people and a free to go life.. what never places him in the Shoes of a powerful King. And, in the Shoes of a man..

He was more like a Vagabond man. A Backpacker somehow.. a showman. Anything esle. The D. Speaks for him. Never chasing anything or anyone.

Now, about the so called "great Kingdom"...If WE look at it, the term "great" refers to a place Not ruled by someone, but animated by someone who was a great man, the greatest version possible of all of them. The Ancient Kingdom refers to something ancient, old enough to be called as it. With Ancient ways, old people, old habits, old casualities. In the End, a place of peace, freedom and calm were everything and everyone does Not really evolves, does Not really move on and develop more and more. The opposite of Human enthropy. The Lands of Gods and Evils. Joyboy was the starting point. But, the thing is that Joyboy could have been living at the same place, was the archetype person as Imu was. But, his opposite Side. One was the representstion of Evil, the other One took Himself for a God-like entity.

The terms "great" does Not necessarily means a "bad/powerful/evil/funky/funny/weird" man. It can be the place of the original members of the D. clans.

No rulers, no Kings, only freedom and careless possibilites of life.

Greatness is something that goes along, to do with caracters, personnality, goofyness sometimes, royalty, Behaviors, approach of people, great care for your loved one's. And even more..

A place Both for good and bad, Gods and evil. And, Imu was After all a man that wished to become a God. He understood that knowing yourself is more powerful with time than being only powerful yourself alone. Because, when a man falls, he becomes Stronger. So, Imu accumulated Knowledge for a very very long period of time before turning Himself into a true Devil, a Kind of devastating man that Always looks a way out of shit and bad situations, realities. A Real time chess payer. And, Joyboy lost the game against Imu.

This is why Joyboy lost the War 800 years Ago. Because Imu's fruit and power are the main source, the origin, Not the Gomu Gomu no mi. Joyboy was maybe the first One to fall and Imu operated the so called "great magic" on Himself and Turned him into a true Devil fruit. Imu had the Knowledge of it all, when Joyboy was the most powerful One, the One true Leader being able to gather people, Make them free and laughing at the same time.

Knowledge versus chaos

Like, a second Zone type of Behaviors, a Deviant One. The One of the D. means, either someone is exploited and Controlled, manipulated.

And, If i Think about Luffy, he is not really a great man of action, in the common Sense and in the perspective of power. He is way too careless. but more like an eternal child that does Not really care about growing UP someday, One day.