Monkey D. Luffy is the protagonist of Eiichiro Oda's One Piece, and he set out to sea with the aim of becoming the Pirate King. While Luffy was a born genius at fighting, he was, expectedly, nowhere near the level of the strongest characters in the story. Luffy's fights against the likes of Aokiji made him realize how big the gap between him and the elites of the One Piece world was. Shortly after, the crew was destroyed at Sabaody Archipelago with Luffy being unable to stop it and all that was followed by the death of his brother, Portgas D. Ace, who Luffy failed to save.
Ace's death became a gateway for Luffy to grow stronger under the tutelage of Silvers Rayleigh, the former right-hand man of the Pirate King. Just like the rest of his crew, Luffy spent two years training his abilities to the fullest. After the timeskip, Luffy has continued to grow stronger in the New World by fighting against strong foes such as the likes of Katakuri, and even the Yonko Kaido. In Wano, Luffy has progressed as a fighter by leaps and bounds and finally established himself as one of the strongest pirates in the whole world, meaning he is potentially close to Yonko level.
What Makes A Yonko
The Yonko are easily the four strongest pirates in the world of One Piece who rule the second half of the Grand Line, called the New World, like kings. Until recently, even the Navy didn't dare to interfere too much into their doings in the New World, however, under Sakazuki's leadership, they're openly challenging the Yonko in the seas. Before the timeskip, their ranks were made up of Whitebeard, Big Mom, Shanks, and Kaido. Whitebeard's death during the Paramount War led to him being ultimately replaced by none other than Blackbeard, the guy responsible for his death.
Every single Yonko in the story is, in some way or form, overpowered. In Whitebeard's case, he was known to wield a Devil Fruit with the power to destroy the world, called the Gura Gura no Mi. Big Mom is a natural disaster who was born with incredible potential that she fulfilled while growing up. Kaido is a pirate known to be the strongest creature alive while Blackbeard wields two of the strongest Devil Fruits in the entire story. Shanks, meanwhile, doesn't have anything special like the rest of the Yonko that the fans know about, however, he is just as strong nonetheless. Being Emperors, every Yonko has great ambition and wants to get their hands on the ultimate treasure, the One Piece. Big Mom and Kaido possess one Road Poneglyph each, Shanks knows where most of the Poneglyphs are, while Blackbeard is surely planning something from behind the scenes.
At the same time, the Yonko also command a massive force which signifies their military might. During the Paramount War, Whitebeard turned up with 1500 members of his own crew, along with 43 ally ships, which just goes to show that he had quite a lot of backing himself. Big Mom's forces number over 10,000 and Kaido has a total of 20,000 members in his crew. Little is known about Shanks but he's likely to have a lot of allies as well. Evidently, the Yonko are more than just strong people and for Luffy to reach their level, he has to fulfill all of these conditions.
Is Luffy Yonko Level?
Luffy's rise to Yonko level began after the timeskip as he gradually grew in terms of strength. During the Dressrosa arc, Luffy showed the fans everything he had in his locker after the timeskip. Whole Cake Island saw him push himself beyond all his limits to surpass Katakuri and, in doing so, learn Future Sight Observation Haki. In Wano Country, Luffy lost to Kaido in a humiliating manner but managed to use this to improve other aspects of his Haki, such as Ryuo, and advance it to the next level. On Onigashima, Luffy finally managed to tap into the power of Conqueror's Haki and matched Kaido in battle, and he's continuing to evolve as the two fight.
In terms of strength, Luffy is already at the level of the Yonko. While he may not have awakened his Devil Fruit, he's taken his Haki to the level of the Yonko, being able to use the advanced form of all three Haki types, which is something that no other Yonko can do at the moment. The biggest indication that Luffy is just as strong as the Yonko came when he started his final one-on-one battle with Kaido and managed to match him blow for blow. The two also clashed their Conqueror's Haki and managed to split the heavens – a feat exclusively achieved by the Yonko in the story so far.
While Luffy may not have an overpowered Devil Fruit or be a born monster, his ambition is greater than any of the Yonko, and he continues to evolve his Haki with time. When it comes to getting closer to the One Piece, Luffy has more Road Poneglyphs than any of the Yonko, meaning he's already a step ahead of them. Furthermore, Luffy has the Straw Hat Grand Fleet at his disposal and the fleet is over 5,000 strong, making him one of the most influential pirates on the seas. Combined with the many allies that he has, such as the Kuja Tribe and even the Samurai of Wano, Luffy's numbers are comparable to the Yonko.
Luffy will also have territories under his protection, just like the Yonko. So far, Fishman Island has already declared for Luffy. It goes without saying that the Land of Wano will be under Luffy's protection once Kaido falls and possibly the likes of Amazon Lily might take his flag as well. In every aspect of piracy, Luffy is just as great as the Yonko of the world. His crew has proven themselves to be capable of fighting other Yonko, with Zoro and Sanji taking down two of Kaido's three right-hand men in Wano and the rest of the crew taking down the Tobiroppo.
While it may be hard for some fans to believe that Luffy is on the same level as the strongest pirates in the story, the narrative proves that he's quite clearly on this level and will, eventually, go on to surpass all of them. As soon as Luffy beats Kaido, he will establish himself as a legendary pirate in the One Piece world and he may gain the biggest bounty after the likes of Roger and Whitebeard as well.