There was hundreds posts about Imu as the final villain and why he shouldn't be final villain, his comparison to Kaguya from Naruto, and that became a reason why proper thread about Imu should exist.
Gorosei was introduced in chapter 233 that called: Those who rules this world.
This chapter was famous all time case it introduced Whitebeard strongest man in the world, Shichibukai and Gorosei. Now from the moment when I saw those old dudes I've got feeling thay possibly could be the final villains in the series. After W7/Ennies Lobby saga this feeling about Gorosei beign the final villains grew even stronger. Ancient Kingdom created One Piece, D. Clan inherited will of people who was citizens or rulers of Ancient Kingdom. And after Marinford war the feeling that the real final enemy is World Goverment became only stronger, case translation of WB last words implied that after finding One Piece where will be war(now that could be wrong translation, but that was feeling I've got there).
I even theorised that there could be hidden king of WG, or someone can became King of WG to became the final villain in this series. But that theory was hell weak. Gorosei even with scars that implied from chapter 233 that they should be fighters, wasn't someone who could take role of final villain. Luffy has to fight the real leader of organization, not just against one of elders. That's the hidden rule of any shonen.
But then we saw Imu first time and that was answer that I was looking for.
Imu the Immortal King of World Goverment, who personaly destroyed Ancient Kingdom and was the reason of One Piece story existance is the real Final VIllain of this series.
Luffy and his crew from Arabasta was fighting against World Goverment, not just pirates. WG was the reason why Shichibukai existance, which caoused the cosequences in face of Crocodile who decided to take over whole kingdom. WG was Roger's main enemy, case WG killed so many people who somehow contacted with Roger, and WG dig ground wil klling even babys to find Roger's son. Akainu and Marines are WG puppets was exist to protect WG. WG was real Luffy's enemy in entire series untill we faced Younko.
Now I know people could be furious because of my next words, but
Younko are nothing more than just fillers. The only reason why they exist was to give Luffy challenge between pirates, to make him great pirate and nothing more. Defeating Younko never was end goal of series.
But there is still question, that should be asked, what about Marshal D. Teach? His name gave you answer already. He is from D. Clan, maybe he is not good guy, but he wants One Piece, his is Luffy's rival, but rival doesn't mean the Final Enemy, depending on series ofcourse. The Main reason why both Luffy and Teach are trying their best to find One Piece is to fullfill inherited will of their ancestors, whi was killed by Imu the King of WG.
SO why then people don't want Imu to be the final villain?
It is simple because of headcanons and desire for Younko to be the strongest in the whole world. Same goes with Admiral fans who don't want for power scaling change so much, that their beloved admiral wouldn't be even in top 10 strongest characters. Each reason creates own agenda. While other readers just want this story to be just a pirate story, due their agenda that pirate story is story when pirates are fighting against pirates. BUT in reality pirate story was always fighing between pirates and military forces of some countries.
Best example here should Pirates of the Caribbean, where after dealing with Davy Jones pirates Will Turner and Jack Sparrow united their forces against Invincible strongest Millirary ship and destroyed it. Basically main enemy of pirates was force like WG, but they was fighing against pirates most of time.
But could same enging work with One PIece? I can Imagine Luffy became the King of Pirates and unites forces against WG, but the something strange starts to happening: Marrines broke in two branches, that created a possibilty that Marrines forces will be not united in the final battle. While due the fact that Luffy will be King he wll summon all his alies from W7 to Wano country, that we met in this series. But for what? To deal with WG the ancient kingdom real enemy.