Imu/Gorosei being final villains destroys like, half of the agendas of ALL types (not just powerscaling) created in One Piece so far. Simple as that. It's like Thanos wiping out half of the universe, Imu just wipes out half of the agendas.
-Yonko fans (I'm a Yonko fan myself):
-Kaido fans: Imu/Gorosei being stronger then Kaido, invalidates the agendas of Kaido fans, some of whom believe his still the strongest man alive in the entire verse. Period. They think Luffy will not fight any body stronger then Kaido ever again (at least on Reddit). Imu destroys that purely via powercreep of being a final villain.
-Blackbeard fans: They seem to have this deep seated obsession with making Blackbeard the final villain. Even though Oda has rubbed their faces in the dirt dozens of times. Like, Law? Running from Shanks? Running from Akainu? Running form old Rayleigh? His been a all around pathetic character that's would be extremely anti-climatic as a final villain and even Oda seems to think so! Blackbeard fans still push so hard to make him the final villain for some reason "he'll kill Imu and become the king of the world so Luffy can fight him at the end" "he'll kill Saturn to get a fruit boost and become stronger then everybody" etc etc. I also think some Zoro fans and Shanks haters also hope to see him as a final villain so they 1 up Luffy, due to Blackbeards many anti feats and Luffy still having him a final villain. They also probably think Blackbeard is the "weakest" final villain Luffy can have, since we already know his much squishier then others, as opposed to Imu having some godly super fruit/s and/or Haki on another level trained for 800 years or something. Imu would be the most frightening for them in that regard, Oda could make Imu absolutely broken capable of no diffing pirate king levels or something insane, and Luffy will rise to that level after he beats Imu.
-Admiral/Akainu fans: They're agenda is already on the ventilator but they've pinned all their hopes on Akainu. Gorosei/Imu being stronger means admirals were just figureheads/servants and they'd be pushed down to glorified Yonko commanders with the Gorosei/Imu being the real deal, like, imagine Franky and Jinbei dealing with GB because Zoro has went to fight Gandhi etc etc.
-Mihawk fans: The Mihawk agenda has taken countless L's, but his fans are still hoping his Zoro's final opponent which is why he'll be strong and not some bum like his being portrayed as. Sword Gandhi Gorosei completely destroys that and Mihawks title as a fraud. It means they also take a massive L on their titlescaling on Shanks. They're current answer to Shanks feats is "but but title WSS". But Gandhi invalidates his title. Without that its just a bum who stalemated and retreated from a YC5 Vs somebody that foddered a admiral/ 1 shotted 1/2 of the ones that defeated BM etc etc.
-Title scalers as a whole: Yup. Mihawk fans think Mihawk is stronger then every person that holds a sword in the verse, Whitebeard fans think Whitebeard is stronger then everybody, Kaido fans think Kaido is stronger then everybody and creature alive etc etc. What happens when a creature (Imu), man (Gorosei), swordsman (Gandhi) all end up stronger? Titlescalers all lose their collective shit. Simple.
-Some Blackbeard fans don't care for power, they just want good, brave, kind, heroic Gary Stu Luffy V sneaky "real pirate behaving", backstabbing dirty trick playing scummy pirate Blackbeard. They remember that bar scene between Luffy and Blackbeard and his dreams speech and think Oda promised them that Blackbeard has to be Luffy's final villain. End of. Its not good enough that he could be a middle villain, still end game, but not final.
The Imu/Gorosei being the final villain murders like, half of the character agendas and most of the powerscaler agendas.