Why Luffy is now a Yonko ?
Being a Yonko isnt just about strenght but influence and territory.
Every Yonko have a major territory in One piece that they control thqnks to their great fleet ans reputation.
Some territories are either conquered or submit to the Yonko do that they can protect them from others pirates and mercenary.
With their reputation alone,Yonkos can this way dissuade with their presence anyone from dare to attack their territories or the ones under their protection.
And if some pirates still despide the warning decide to still attack the territories under the yonko, they will face the wrath of the strongests pirates who can easily destroy Islands, cause earthquake and sea to rise and Split the Sky.
Each Yonko have a powerfull army that consist of the main crew and their allies from around the World. In others World its a huge organisation with leader the Yonkos who rule the sea.
Now, back to Monkey D Luffy.
Luffy through his aventure in One piece, already delivered many kingdom and Islands and got their freindship.
In Dressrosa, Luffy got many allies which end UP in the creation of Strawhats grand fleet.
The fleet is made of renewed pirates of the New World and even Old legend.
Luffy also has many territories under his protection like Wano who see Luffy as a hero After he defeated the forme Yonko Kaido the World Strongest Creature a wisdom dragon.
Luffy also posess one of the strongests crew in the World. The commenders of the strawhats are powerfull idividual who aspire to their own goal allongside their captains.
Each if them defeated the top commenders of The Beasts pirates crew.
Luffy becoming a Yonko isnt just because he defeated Kaido otherwise Kidd and Law were good contenter toi cause they defeated Big Mom and got same bounty as him.
His territories, reputation as a pirates around the World, the crew he gathered, and the threat he represent as Nika Joyboy to the World gouvernement are the reasons he become a Yonko.
Ulike him , Kidd, Law dont have territories, a crew as strong.
If anything Luffy's crew can take out both Law and Kidd pirates qt thé same Time.
They are that strong, with powerfull Swordman like Zoro who is in top tier territory no After he Awakened his conqueror and learned in batle how to cloack it in his Swords to increase his damage tremendously.
Luffy isnt only a Yonko, He is at least the number 3 (buggy being Yonko thank to put luck After Mihawk joined Cross Guild) and very Close to Shanks After defatinf Kaido.
And its a good news After 1000 chapters of d'aventure it was about Time for Luffy to reach that statue and get Closer to his Dream which is to be pirates King.
Luffy also has already 3 of the road poneglyph with only one missing. And with Robin, Luffy can easily reach the final destination for the one piece After defeating Blackbeard and proving his superiority for all as strongest Pirates.